matrix-addressable micropixellated ingan light-emitting diodes with uniform emission and increased...

2650 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 54, NO. 10, OCTOBER 2007

Matrix-Addressable Micropixellated InGaNLight-Emitting Diodes With Uniform Emission

and Increased Light OutputZ. Gong, H. X. Zhang, E. Gu, C. Griffin, M. D. Dawson, Senior Member, IEEE,

V. Poher, G. Kennedy, P. M. W. French, and M. A. A. Neil

Abstract—Micropixellated InGaN light-emitting diodes (micro-LEDs) have a wide number of potential applications in areas in-cluding microdisplays, fluorescence-based assays and microscopy,and cell micromanipulation. Here, we present fabrication and per-formance details of matrix-addressable micro-LED devices whichshow significant improvements over their earlier counterparts.Devices with 64 × 64 micropixel elements, each of them havinga 16-µm-diameter emission aperture on a 50-µm pitch, have beenfabricated at blue (470 nm), green (510 nm), and UV (370 nm)wavelengths, respectively. Importantly, we have adopted a schemeof running n-metal tracks adjacent to each n-GaN mesa, so thatresistance variation between the devices is reduced to below 8%,in contrast to the earlier fivefold resistance variation encountered.We have also made improvements to the spreading-layer forma-tion scheme, resulting in significant increases in output powerper element, improved current handling, and reduced turn-onvoltages. These devices have been combined with a computer-driven programmable driver interface operating in constant-current mode, and representative microdisplay outputs arepresented.

Index Terms—InGaN, micropixellated light-emitting diode.

I. INTRODUCTION

M ICROPIXELLATED InGaN light-emitting diode(micro-LED) arrays have attracted much interest

recently due to their wide-ranging prospective uses in areassuch as microdisplays [1]–[7], fluorescence-based imagingand measurement [8], spectral conversion [9], and mask-freelithography [10]. For such applications, it is highly desirablethat these devices have the largest possible optical output powerper pixel, high brightness and uniform emission capability, aswell as a high degree of spatial, spectral, or temporal control.To fulfill these requirements, micropixellated LEDs have beendeveloped, where the emission area is patterned into high-density arrays of microemitting elements in forms includingdisks [1]–[3], [5], [11], rectangles [4], rings [6], stripes [7], andhexagons [12]. These devices are readily integrated with other

Manuscript received April 5, 2007; revised July 10, 2007. The review of thispaper was arranged by Editor L. Lunardi.

Z. Gong, H. X. Zhang, E. Gu, C. Griffin, and M. D. Dawson are withthe Institute of Photonics, University of Strathclyde, G4 0NW Glasgow, U.K.(e-mail: [email protected]).

V. Poher, G. Kennedy, P. M. W. French, and M. A. A. Neil are with theDepartment of Physics, Imperial College London, South Kensington Campus,SW7 2AZ London, U.K.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2007.904991

components, such as polymer microlenses, for beam projection[2], [10].

One of the technical challenges in fabricating such micro-LED arrays at high element density is in developing aninterconnect scheme to address each pixel separately. A matrix-addressing scheme, where all pixels along one column sharea common anode (cathode) and all pixels along one rowshare a common cathode (anode), is effective in this regard[2]–[5], [11]. With this scheme, only 2N contacts are requiredfor N × N LED arrays, much less than those required usingan individual addressing scheme (2N2). However, in our ex-perience, these matrix-addressable LEDs (referred to in whatfollows as “conventional devices”) have emission nonunifor-mity issues, which have not attracted much attention to date.Furthermore, these matrix-addressable LED arrays are proneto have a large series resistance and a limited optical outputpower, which may restrict practical device applications. Someresolutions to these problems have been preliminarily reportedin [13]. Here, we present these solutions in much greater detail,together with a range of new results. First, we systematicallyinvestigate the main causes for nonuniformity in the conven-tional matrix-addressable LED devices. This is achieved byestablishing a simple analytical model. Then, we optimize thedevice structure with the aid of this model and develop newprocesses to improve the device performance. These includethe following: 1) an additional side-rail n-metal line, whichis introduced to suppress the resistance variation along then-GaN mesa and thus to improve the emission uniformityacross the LED array [11], [13]; 2) a new spreading-layerformation scheme, which is proposed to decrease the contactresistance and to enhance the light output of each pixel; and3) some key processing techniques, for instance a ring-shapedp-contact, which are introduced in the new device to overcomethe difficulty of exposing the thin spreading layer. Subse-quently, we characterize the performance of the new devicesin full detail in terms of I–V characteristics, series resistance,emission uniformity, and light output and compare them withtheir earlier counterparts. Finally, a suitable driver is designed,and highly uniform microdisplays are demonstrated.

II. STRUCTURE AND EMISSION CHARACTERISTICS

OF THE CONVENTIONAL DEVICES

Fig. 1(a) and (b) shows a cross-sectional schematic anda plan-view optical micrograph of a conventional device,

0018-9383/$25.00 © 2007 IEEE

GONG et al.: MATRIX-ADDRESSABLE MICRO-LEDs WITH UNIFORM EMISSION AND INCREASED LIGHT OUTPUT 2651

Fig. 1. (a) Schematic structure of the conventional LED arrays, (b) opticalmicroscope image of the as-fabricated conventional device, and (c) emissionnonuniformity of a row of pixels in the conventional LED arrays. The biasvoltage applied to each pixel is 4.2 V (dc operation).

respectively. The fabrication details of these devices have beenreported in [5]. The emission characteristics of such a deviceoperating at blue wavelength (∼470 nm) are shown in Fig. 1(c).For clarity, only those LEDs in the same row (i.e., along ann-GaN mesa) are connected in a parallel fashion. While thelightness variation within a specific pixel is hardly observable,the lightness difference among those pixels along the n-GaNlength is quite distinct. Some pixels close to the end of then-GaN mesa are not even turned on under the same forwardvoltage (4.2 V, dc operation). By analyzing the correspondingI–V curves for these LED pixels, we find the series resistanceof each pixel to differ, increasing with an increase in the n-GaNlength. The turn-on voltage, depending on the specific pixelposition, also varies from 4.0 to 4.5 V. All of these results revealthat noticeable emission nonuniformity occurs along the row,which is undesirable if these devices are to be used for practicalapplications.

III. ANALYTICAL MODEL FOR THE

EMISSION NONUNIFORMITY

Basically, there are two types of emission nonuniformity,which are worthy to note. The first one is related to the lateralcurrent variation within each specific pixel. The second one, onthe other hand, refers to the average lightness variation amongthose pixels in the same row. In this section, we will analyticallydemonstrate that the first kind of emission nonuniformity isapproximately negligible. We then focus on establishing asimple model to analyze the second type of emission nonunifor-mity, i.e., lightness variation among different pixels along thesame row.

According to [14] and [15], the current density of a LED witha lateral injection geometry can be expressed by

I(x) = I(0) · exp(−x/Ls) (1)

where I(0) is the current density at the edge of the mesa, x isthe distance from the edge of the spreading layer, and Ls is the

Fig. 2. Effect of the current spreading length (Ls) on the current variationfactor (f) for different device dimensions (d). The corresponding currentvariation factor for the device with a lateral dimension of 20 µm is indicatedwith a dark point in the figure.

Fig. 3. (a) Schematic of one row of a conventional InGaN micro-LED arrayand (b) equivalent circuit model for each pixel.

effective current spreading length, which is given by [14]

Ls =

√ρc + ρptp

|ρn/tn − ρt/tt|(2)

where ρc is the specific contact resistance of the p-contact; ρp,ρn, tp, and tn are the resistivities and thicknesses of the p-and n-type cladding layers. ρt and tt are the resistivity and thethickness of the semitransparent spreading layer, respectively.Therefore, the current variation factor f across a LED with alateral dimension of d is given by

f =I(0) − I(d)

I(0)= 1 − exp(−d/Ls). (3)

Fig. 2 shows the calculated current variation factor f as afunction of the current spreading length Ls for different devicedimensions d. Apparently, under a fixed current spreadinglength Ls, a smaller device dimension d favors a more uniformcurrent distribution within the device.

A schematic structure of conventional micro-LED elementsin one row is shown in Fig. 3(a), where the n-contact is locatedat the left side, while the p-contacts are distributed on top ofeach respective pixel at an equal spacing. In this conventional


TABLE IPARAMETERS USED FOR CALCULATING THE CURRENT

SPREADING LENGTH OF MICRO-LEDs

design, the n-GaN mesa in the structure serves as a conductivepath for each pixel. Following [14], the calculated currentspreading length Ls for the blue device here is around 210 µm.1

The parameters used in the calculation are listed in Table I. Con-sidering here that the pixel diameter (20 µm) is much smallerthan the calculated current spreading length, the maximum cur-rent variation within one pixel is thus quite small (∼9.1% theo-retically, as indicated by the dark point in Fig. 2). Therefore, wecan approximately assume that the current spreading is uniformwithin such a small pixel. This assumption is in reasonableagreement with our experimental results, which enables us toonly consider the lightness differences among different pixels inthe same row (i.e., the second type of emission nonuniformity).

We then treat the n-GaN mesa (between the n-contact padand the target pixel) as a resistance in series with the LED pixel.Thus, the equivalent circuit of each pixel can be simplified inthe form shown in Fig. 3(b). In terms of this circuit model, theseries resistance Rs for each pixel can be expressed as a sum ofvertical resistance Rν and lateral resistance Rl, i.e.,

Rs = Rν + Rl (4)

where Rν is the sum of p-contact resistance Rpc, p-cladding-layer resistance Rp, and n-contact resistance Rnc. That is

Rν = Rpc + Rp + Rnc (5)

and Rl is the lateral resistance of the n-GaN mesa, which isdefined by

Rl = ρ · L/S = Rsc · (L/W ). (6)

Here, ρ is the resistivity of the n-GaN mesa, L denotes thedistance between the common n-contact and the target pixel (orthe length of the n-GaN mesa), S is the cross-section area of

1Note that Ls is a parameter independent of the device geometry. Assumingthat the current distribution within each pixel is 1-D (along the n-GaN row),then (2) is still valid despite the fact that, here, each pixel has a circulargeometry instead of a rectangular geometry [14], [15].

the n-GaN mesa, Rsc is the sheet resistance of the n-GaN mesa,and W is the width of the n-GaN mesa. Inserting (6) into (4)yields

Rs = Rν + Rsc · (L/W ). (7)

For each pixel, the vertical resistance Rν should be equiv-alent. However, the lateral resistance Rl is linearly dependenton the distance L. Hence, based on (7), we conclude that theseries resistance of each pixel will linearly increase with thedistance between the n-contact and the pixel (L). If Rl is largeenough, most of the voltage applied will drop at the n-GaNmesa rather than the LED pixel. Thus, a high drive voltage willbe required to turn on the target pixel far from the n-contact,which is particularly undesirable if a standard CMOS circuit isused to drive such high-density micro-LED arrays.

An important conclusion can be derived from the simplemodel above. That is, to ensure uniform emission, the devicestructure should be optimized so that Rl is minimal.

IV. DEVICE DESIGN AND FABRICATION

Based on the above analysis, we redesigned the device struc-ture in order to improve the performance. A key alteration inthe new device structure is that an additional n-contact metalline is inserted along the whole n-GaN mesa (an apparentlysimilar strategy has been adopted by Wu et al. [11] but notcharacterized in detail). We anticipated that this metal linewould effectively increase the conductivity and would suppressthe lateral resistance variation with the n-GaN length. In addi-tion, we adopt a new strategy for the formation of the currentspreading layer (see below) with the aim of improving theohmic contact of the spreading layer to p-GaN.

The devices were fabricated from standard metal–organicchemical-vapor deposition-grown LED wafers, with wave-lengths ranging from green (510 nm) and blue (470 nm) toUV (370 nm). Full details of the wafer structures can be foundelsewhere [3]–[7]. The basic fabrication requirement is for thepixels within each row to be electrically isolated from thosewithin other rows, which is realized by etching rectangularmesa structures down to the sapphire substrate. All pixels inthe same row share a common n-electrode line alongside themesa, and all pixels in the same column are interconnected bya common p-contact line running across the parallel rows. Inthis way, a matrix-addressing scheme is enabled. Processingof the new device is slightly more complicated than that ofthe conventional devices [5]. Normally, six masking steps wererequired to complete the new device. The first step defines theGaN mesa (row) by inductively coupled plasma (ICP) etching,which has a sloped sidewall [5] and thus allows conformalmetallization. The second step defines each pixel in circulardisk geometry, which is also accomplished by ICP etching.The next two masking steps are for the metallization of thespreading layer (Ni/Au, 3 nm/9 nm, thermally evaporated) andn-contact metal lines, 4 µm in width, running along each row(Ti/Au, 20 nm/120 nm, sputtered), as well as a ring-shapedcontact on the spreading layer. Then, a 200-nm-thick silicon-oxide layer is deposited onto the surface to act as an isolation


Fig. 4. (a) Schematic structure of the new device and (b) correspondingoptical microscope image.

layer. The following step is to expose the n-contact pads and thering-shaped contact on top of the spreading layer by reactiveiron etching (RIE) using photolithography-defined patterns asmasks. Finally, the interconnection of each pixel is performedby Ti/Au metal lines (20/120 nm) using a sputtering process.

Fig. 4 schematically shows a structural diagram of the newdevice (a) and a microphotograph (b) of the individual n-contact(p-contact) arrangement to each row (column). Also clearlyobservable is the inserted metal line along the n-GaN mesa. It isnoteworthy that the dimensions of each mesa and element of thenew device are also appropriately adjusted so as to accommo-date the metal lines. However, the pixel number, the effectiveemission area of each LED pixel, and the overall device sizeare kept identical to the conventional devices (this is achievedby appropriately reducing the lengths of both p-contact and n-contact pads). Detailed dimensions of the conventional devicesand the new designed devices are summarized in Table II.

The role of the ring-shaped contact on the spreading layershould be emphasized here [see Fig. 4(a)]. As already statedabove, subsequent SiO2 isolation necessitates the spreadinglayer to be exposed for interconnection. However, directlyexposing the spreading layer by RIE is a great challenge as itis only 12 nm thick (the well-established wet-etch method isnot applicable as buffer-oxide-etch (BOE) agent can erode Niand thus destroy the spreading layer). To ensure that the siliconoxide on the spreading layer is completely removed, a dry

TABLE IIDIMENSIONS OF THE CONVENTIONAL DEVICE

AND THE NEW DEVICE FORMATS

Fig. 5. Typical I–V characteristics of the conventional device and the newdevice, both emitting at 370 nm.

overetch strategy is adopted. However, overetch will induce se-rious plasma damage and will partly destroy the thin spreadinglayer, resulting in a poor ohmic contact. Therefore, we designeda thick ring-shaped contact on top of the spreading layer toact as an etch sacrificial structure. Then, we expose the ring-shaped contact by RIE, rather than exposing the spreading layerdirectly. Hence, the thin spreading layer will not experienceany plasma damage (although part of the emitted light is thenblocked by the ring-shaped contact).

V. DEVICE PERFORMANCE

Fig. 5 shows typical current–voltage (I–V ) characteristics ofa single pixel at an identical position in the new UV micro-LEDarrays and conventional UV arrays emitting at 370 nm, respec-tively. As it is evident, by introducing metal lines along eachn-GaN mesa and the new spreading-layer formation scheme,the I–V characteristic of the new UV device is substantially


Fig. 6. Microscope image of a new blue device showing two rows of LEDsturned on along the n-GaN mesa. The bias voltage applied to each pixel is4.0 V (dc operation).

improved: the turn-on voltage is only 4.8 V, and the bias voltageat 1 mA is 5.2 V. In contrast, the turn-on voltage and thebias voltage at 1 mA of a typical conventional device were5.7 and 7.1 V, respectively. Similarly, we find that the I–Vcharacteristics of the new green and blue devices are alsoimproved (not shown here).

Fig. 6 shows the emission characteristics of LEDs fabricatedby the new method (for purposes of illustration, a blue devicewith an emission wavelength of 470 nm is shown here). We cansee that the emission uniformity across the whole row is greatlyimproved compared with that of the conventional device [seeFig. 1(c)]; no substantial difference in the emission intensity isobserved by the naked eye. Although there is optical reflectionat the sidewall, the light emission from each pixel shows a well-defined circular pattern as shown by the magnified image.

In order to further understand the improved emission uni-formity, we have measured the series resistance of each pixelas a function of the pixel position (or the n-GaN length). Theseries resistance of LEDs can be obtained from the slope of thefollowing equation [16]:

IdV

dI= I · Rs + nKT/e (8)

where n is the ideality factor, K is the Boltzmann’s constant,and Rs is the series resistance (when V > Vturn−on, Rs ≈(dV/dI)).

Fig. 7 shows the measured series resistance of each pixelas a function of the n-GaN length (or pixel position). For theconventional blue device, the series resistance of each pixelincreases linearly from 2.4 to 10.7 kΩ, showing a nearly fivefoldincrease in resistance from the start to the end of the n-GaNmesa, as shown by the closed-circle data. To separate the resis-tance contribution from the n-GaN mesa, we fit the data of themeasured series resistance with (7). The fitted sheet resistanceof the n-GaN is 105 Ω/sq (this result is in reasonable agreementwith the sheet resistance of 96 Ω/sq measured by transmission-

Fig. 7. Series resistance of the conventional device (closed circles) and thenew device (closed triangles), and the calculated lateral resistance (closedsquares) as a function of the n-GaN length.

line methods). Thus, the lateral resistance contributed fromthe n-GaN mesa is 131 to 8297 Ω, depending on the specificpixel position (see closed-square data). Note that the verticalresistance Rν (the fitted value) is only 1.89 kΩ. This indicatesthat the lateral resistance is indeed nonnegligible as long as thelength of the n-GaN is large enough, which is in agreementwith the prediction of the model that we proposed earlier. Thisalso means that the conductivity of the n-GaN mesa alone,although it is highly doped, is not sufficient to provide a low-resistance path to the LEDs close to the end of each mesa. Thegradual decrease in the emission intensity of each pixel, withthe position shown in Fig. 1(c), reinforces our conclusion.

For the new blue device, however, after the metal linesare inserted, the series resistance of each pixel only slightlyincreases from 560 to 601 Ω (see closed-triangle data). Theseresults indicate that the inserted n-metal lines can effectivelysuppress the resistance variation of each pixel along the n-GaNmesa and can reduce the total series resistance of each pixel. Forthe new blue device we discussed here, the resistance variationalong the n-GaN length is smaller than 8%. The suppressedresistance variation in turn improves the emission uniformityof each pixel in the row, as shown in Fig. 6.

After introducing the metal lines, the lateral resistance con-tributed from the n-GaN is negligible, and the series resistanceof the new device can be expressed as

R′s = R′

metal−line + R′nc + R′

pc + R′p (9)

where R′metal−line, R′

nc, R′pc, and R′

p are the resistance ofthe inserted metal line,2 the n-contact resistance, the p-contactresistance, and the p-cladding-layer resistance, respectively. Toget an analytic expression of (9), we assume that the current

2As the inserted metal line is very small and thin, the bulk resistance cannotbe neglected anymore.


spreading is uniform under the n-contact.3 Then, (9) can betransferred into the following one:

R′s = ρl ·

(L

W ′

)+

(ρnc

A0 + L · W ′

)+ R′

pc + R′p. (10)

Here, ρl and W ′ are the sheet resistance and the width of theinserted metal line, respectively. ρnc and A0 is the specificresistance and the original size of the n-contact, respectively.Considering that the dimension (A0 = 0.0035 cm2) of then-contact pad is much larger than that of the inserted metal line(4 µm in width and 3200 µm in length), (10) can be furthersimplified into

R′s∼= ρl ·

(L

W ′

)+

(ρnc

A0

)+ R′

pc + R′p = ρl ·

(L

W ′

)+ R′

ν

(11)

where

R′ν =

(ρnc

A0

)+ R′

pc + R′p = R′

nc + R′pc + R′

p. (12)

Note that R′ν is a parameter independent of the n-GaN length

(L). Therefore, the slight series-resistance variation for the newdevice is mainly caused by the bulk resistance change of themetal line with its length.

Similarly, we fit the measured series resistance by using (11),and we get ρl = 0.05 Ω/sq and R′

ν = 560 Ω. Therefore, theresistance contribution from the metal line is about 0.25−40 Ω.Considering that this value is far smaller than that of the con-ductive n-GaN mesa (131 to 8297 Ω), the current will mainlyflow via the metal line instead of the n-GaN mesa (that is, then-GaN resistance is shunted in the new device). This accountsfor the suppressed resistance variation with the n-GaN length.

Note that the vertical resistance of the new device (R′ν =

560 Ω) is much smaller than that of the conventional device(Rν = 1.89 kΩ). Considering that the contact area A0 is quitelarge, Rnc(∼= R′

nc) is approximately negligible. Thus, the verti-cal resistance ratio R′

ν/Rν is given by

R′ν

Rν=

R′pc + R′

p + R′nc

Rpc + Rp + Rnc

∼=R′

pc + R′p

Rpc + Rp. (13)

Assuming that the current distribution is uniform within thepixel,4 then (R′

ν/Rν) can be also expressed as

R′ν

Rν=

ρ′c + ρptpρc + ρptp

· π · r2

π · r′2 (14)

where r and r′ are the radii of each pixel in the conventionaldevice and the new device, respectively. ρ′c is the specificp-contact resistance of the new device. Noting that ρ′c

3Strictly speaking, the contact resistance is heavily dependent on the currentdistribution and the contact geometry and cannot be solved analytically inour case.

4This assumption should be approximately reasonable according to theanalysis in Section III.

Fig. 8. Output-power density of the devices emitting at (a) 470 nm and(b) 370 nm as a function of current density.

ρptp, and ρc ρptp, the above equation can be furthersimplified into

R′ν

Rν=

ρ′cρc

· r2

r′2. (15)

From (15), we then conclude that the smaller vertical resis-tance of the new device is mainly attributed to the increasedpixel radius and the improved contact quality. For example, thecalculated vertical resistance ratio for the blue device is 0.2,which is comparable to our experimental result (0.31).5

A higher power density for the new blue devices has alsobeen achieved, as shown in Fig. 8(a). The power data aremeasured from the same pixel as discussed earlier. For thenew blue LED array, the output-power density of each pixel is3.65 W/cm2 at a current density of 300 A/cm2, which is nearlythree times larger than that of the conventional blue device(1.25 A/cm2). Similarly, the output-power density of each pixelin the UV array is also substantially enhanced, as shown inFig. 8(b). It is noteworthy that the new UV array can nowoperate under a much higher current density of ∼2000 A/cm2

(in contrast, the conventional UV device can only operate underthe injection-current density smaller than 700 A/cm2) and thuscan provide a higher output-power density (∼6 W/cm2), asshown in Fig. 8(b). Similarly, both the new blue and the greenLED arrays can work under a higher current density (voltage).

5For a more accurate calculation, however, it is essential to take into accountthe geometry of the contact area and the current spreading.


The larger power (density) output of the new device canbe mainly attributed to the higher light-extraction efficiency,which can be externally enhanced by modifying the devicestructure and improving the transparency of the spreading layer.Here, we have adopted an improved formation scheme for thespreading layer in the new device, which should favor thelight extraction. In our original device, the spreading layerwas formed by evaporating a Ni/Au bilayer into small holeswhere SiO2 is removed by BOE etching agent. However, someresidual SiO2 still remains on the open area, resulting in apoor and nonuniform contact to p-GaN and thus poor lightoutput. For the new device, a Ni/Au thin spreading layer isdirectly deposited onto the exposed p-GaN area defined by aphotoresist pattern prior to the SiO2 deposition. This guaranteesthe formation of a high-quality transparent electrode, permittingmuch more uniform current injection and higher light output. Itshould be noted that the remaining SiO2 on top of each pixel inthe new LED array also favors the increase of light extraction(see Fig. 4). Due to the fluctuating nature of the spreading layerand the uneven surface of the aperture, a naturally textured SiO2

surface is formed on top of each pixel (not shown here). It iswell known that surface texturing can effectively reduce theloss of photons due to the internal reflection [17]. Furthermore,considering the fact that SiO2 has a smaller refractive indexthan GaN, the probability of photons escaping from the SiO2/airinterface should be higher than from the GaN/air interface [18].All these factors can lead to the higher extraction efficiency and,thus, the higher output for the new device. Finally, consideringthat the series resistance of the new device is substantiallydecreased with the introduction of the n-contact line, the heateffect should be greatly suppressed and, thus, should favor thelight extraction at high current density. This is in reasonableagreement with our measurement results, as shown in Fig. 8(b).

It should be noted that the output power of the new devicewould have been much higher if part of the light were notblocked by the additional ring-shaped p-contact (see Fig. 4).This problem can be potentially resolved by adopting flip-chipLED structures (that we are currently exploring) in which athick reflective p-contact layer covering the entire spreadinglayer will be used instead of the ring-shaped contact. Pre-liminary results indicate that the power output can be furtherenhanced by several times in this way, but this is beyond thescope of this paper and will be reported elsewhere.

VI. MICRODISPLAYS

To demonstrate the use of these devices as simple microdis-plays, we have designed suitable driver circuitry, which canbe externally programmed via a personal computer interface.Fig. 9 shows a diagram of the electrical driver. The matrix isaddressed by applying a positive bias voltage to a single rowacross the columns at a rate up to 38 000 rows per secondusing MIC5891 programmable voltage sources. In this way,the driver is capable of turning on one row at a time, then itmoves on to the next row in quick succession. In addition, tocompensate the remaining small variation among the pixels, theLED pixels within the selected row are driven in a constantcurrent mode using MAX6971 constant-current sink drivers

Fig. 9. Diagram of the driver circuit for 64 × 64 micro-LED arrays.

Fig. 10. Representative emission patterns programmed onto the micro-LEDarrays emitting at (a) blue, (b) green, and (c) UV wavelengths.

for each column electrode. The overall effect is to producea binary time-integrated pattern, which is flicker-free as longas the refreshing rate is fast enough. The emitter drive cur-rent is externally adjustable (from 4 to 50 mA), and a 10-bitpulse-wave-modulation control is used to allow fine brightnesscontrol of the display. Patterns can be generated in softwareusing a Labview graphical interface and are sent from the hostcomputer to the driver board using a USB communication port.


Thus, any pattern can potentially be displayed on the 64 × 64matrix at up to 600 frames per second.

As an illustration of this capability, three images displayed onthe LED arrays with different emission wavelengths (i.e., blue,green, and UV) are shown in Fig. 10. As we can see, the overallemission uniformities across the whole array are very good forthese LED arrays, apart from some “point defects” with ex-ceptionally strong luminescence or no luminescence, which arebelieved to be caused by localized processing, epi-defects, wire-bonding faults, and/or metal step-coverage problems. Also, wewould emphasize that the emission uniformity of all these newdevices is much better than those of the conventional devices.For the conventional LED arrays, because of the electricalresistance variation induced by the n-GaN mesa, it is hard tocompensate the difference of emission intensity of the pixelseven when a constant current driver is used. Particularly for theconventional UV devices, due to the high turn-on voltage, theyare unable to be operated until a high bias voltage is applied.

VII. CONCLUSION

A simple model was established to analyze the origin ofthe emission nonuniformity of matrix-addressable micro-LEDarrays. This model was used subsequently to aid in revis-ing the device structure and thus to improve the device per-formance. By inserting an additional metal line along eachn-GaN mesa, the emission uniformity of the micro-LED arrayswas substantially improved, thanks to the suppression of theresistance variation. Furthermore, by adopting a new p-contactformation scheme, the optical light output was enhanced. Otherimproved characteristics included a lower turn-on voltage anda smaller forward voltage. Using a simple driver circuit, high-quality microdisplays were demonstrated. The success of thesedevices opens up a wide range of potential application in otherareas such as biosensing and microinstrumentation.

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Z. Gong received the Ph.D. degree in condensed-matter physics from Institute of Semiconductors,Chinese Academy of Sciences (CAS), Beijing,China, in 2005, where he mainly focused on themolecular-beam-epitaxy growth and characterizationof low-dimensional semiconductor quantum struc-tures such as In(Ga)As/GaAs quantum wires, quan-tum dots, and quantum rings.

In 2005, he received a Liuyonglin Special Awardfrom CAS. Since May 2005, he has been with theInstitute of Photonics, University of Strathclyde,

Glasgow, U.K., as a Research Fellow, becoming involved in the fabrication,characterization, and application of GaN-based micro-LED arrays.

H. X. Zhang received the Ph.D. degree fromZhejiang University, Hangzhou, China, in 2001,working on the growth and characterization of GaNon Si substrate.

After receiving the Ph.D. degree, he was em-ployed as a Postdoctoral Research Fellow at theCenter for Quantum Devices, Northwestern Univer-sity, Evanston, IL, working on the growth, charac-terization, and processing of III-nitride UV LEDsand laser diodes (280–350 nm). He has been withthe Institute of Photonics, University of Strathclyde,

Glasgow, U.K., since April 2005. His current research at the institute focuses onthe development of (InGa)N/(AlGaIn)N micro-LEDs. His interest covers a widerange in epitaxial growth, characterization, and processing of III–V compoundsemiconductors.


E. Gu received the Ph.D. degree in thin-film physicsfrom Aberdeen University, Aberdeen, U.K., in 1992.

He was appointed as a Research Fellow atCavendish Laboratory, Cambridge University, wherehe worked on epitaxial magnetic-film growth, insitu structural and magnetic property characteriza-tions of epitaxial films, micromagnetic structures,and thin-film devices. In December 1997, he joinedthe thin-film group, Oxford Instruments plc, U.K.,as a Senior Engineer. His work in Oxford Instru-ments focused on superconducting device research

and development such as single-electron tunneling devices, superconductingphoton detectors, and transition-edge sensors. Since 2002, he has been withthe Institute of Photonics, University of Strathclyde, Glasgow, U.K., as aResearch Team Leader. His research interests include epitaxial growth, in situand ex situ characterizations, process developments, and device fabrication andcharacterization.

C. Griffin received the degree in physics (with hon-ors) from the University of Aberdeen, Aberdeen,U.K., in 2000 and the Ph.D. degree from Institute ofPhotonics, University of Strathclyde, Glasgow, U.K.,working on the applications of micropixellated III–Vsemiconductor nitride LED arrays.

After completing his first degree, he worked forMarconi for two years as a Hardware DevelopmentEngineer, testing and designing telecom equipment.Then, since obtaining his Ph.D. degree, he hasbeen with the Institute of Photonics, University of

Strathclyde, as a Postdoctoral Researcher. His research concentrates on theoperation, characterization, and applications of micron-scale AlInGaN/GaNand InGaN/GaN LEDs emitting in the near UV to blue wavelength range.

M. D. Dawson (SM’98) received the B.Sc. degreein physics and the Ph.D. degree in laser physics fromImperial College London, London, U.K., in 1981 and1985, respectively. His thesis work covered opticalgain switching in semiconductor lasers and simul-taneous mode locking and Q-switching in Nd:YAGlasers.

From 1985 to 1991, he was a Visiting AssistantProfessor in Professor A. L. Smirl’s group, first atNorth Texas State University and subsequently atthe University of Iowa, working on the development

of femtosecond dye lasers and the application of these sources to ultrafastand nonlinear spectroscopy of III–V semiconductors. From 1991 to 1996, heheld the position of a Senior Researcher at Sharp Laboratories of Europe,Ltd., Oxford, U.K., performing optical spectroscopy on AlGaAs and AlInGaPsemiconductor heterostructures. He is currently a Professor and an AssociateDirector with the Institute of Photonics, University of Strathclyde, Glasgow,U.K., where he founded and leads the III–V semiconductor materials and de-vices programme. His research interests cover semiconductor thin-disk lasers,dilute (GaInNAs) and wide-bandgap (AlInGaN) nitride materials and devices,and microfabrication in a range of hard and soft optical materials. He is theauthor of over 350 publications in journals and conference proceedings.

Dr. Dawson is a Fellow of the U.K. Institute of Physics, the Optical Societyof America, and the Royal Society of Edinburgh.

V. Poher, photograph and biography not available at the time of publication.

G. Kennedy, photograph and biography not available at the time of publication.

P. M. W. French, photograph and biography not available at the time ofpublication.

M. A. A. Neil, photograph and biography not available at the time ofpublication.

matrix-addressable micropixellated ingan light-emitting diodes with uniform emission and increased...

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