3.5. Experimental Results 83

3.5.2 L–I–V Measurements

The characteristics of devices from the different samples after the front-side processingare compared. For the devices with recess etch the results after the substrate thinningand anti-reflection coating deposition will be shown as well.

The devices are characterized by detailed light, current and voltage (L–I–V) mea-surements. The set-up consists of a calibrated large area silicon photodiode (Hama-matsu S1337-1010BR) and a HP 4145A Semiconductor Parameter Analyzer which isused as DC voltage source, current monitor and photocurrent monitor simultaneously(see figure 3.18). All measurements are taken in cw (continuous wave) mode and atroom temperature. The device is contacted with two prober needles connected viacoaxial cables. A voltage is applied to the diode and is varied over a certain range whilethe resulting diode forward current and the photocurrent generated in the photodiodeare recorded. The photodiode is biased at 0 V in order to minimize its dark current.

Bottom emitting devices are measured simply by placing them either on a glassmicroscope slide on the photodiode or directly on the photodiode. As the surface ofthe photodiode of 1 cm2 is large compared to the device sizes, it is assumed that all theemitted light is captured by the photodiode if the device is centered on it.

The external quantum efficiency ηext is defined as the ratio of the externally emittedphoton flux, Φout

opt, to the total injected electron flux, Φtotel (see section 2.5.1)

ηext =Φout

opt

Φtotel

=e

hν

Popt

I(3.13)

where hν denotes the energy of the emitted photons, e the electron charge, Popt the

HP 4145A

SPA

Photodiode

MCLED

Prober

needles

+-

A

COMMON

A

Figure 3.18: Schematic representation electrical circuit L–I–V measurement set-up

84 CHAPTER 3. Bottom Emitting MCLEDs at 970 nm

Figure 3.19: Typical spectral response large area photodiodes by Hamamatsu

optical output power and I the injected current [37]. The measured external quantumefficiency is therefore calculated as follows:

ηext =λ[µm]

1.24

IPD

RPDIF

(3.14)

RPD corresponds to the photo sensitivity of the photodiode, which is expressed in unitsof A/W. The typical spectral response of the photodiodes S1337-1010BR and S1337-1010BQ given by Hamamatsu is shown in figure 3.19. The photodiodes S1337-1010BQhave a quartz window instead of a resin coating and show therefore an increased sensitiv-ity below 400nm but a reduced sensitivity for longer wavelengths. All the measurementswere carried out with a photodiode S1337-1010BR, which had been calibrated by theSwiss Federal Office of Metrology and Accreditation (METAS). The calibration resultsare listed in appendix D.1.

The external quantum efficiency of the photodetector, which corresponds to theproduct 1.24RPD/λ[µm], is equal to 0.786 at 970nm. As the detector quantum efficiencyis more or less constant in this wavelength regime this value is used for the deviceefficiency calculation. The detector efficiency is assumed to be independent of the angleof incidence.

ηext =1

0.786

IPD

IF

(3.15)

Since the LED characteristics at low current densities are representative for theelectrical and optical quality of the device, the current density–voltage curves are plottedon a semilogarithmic scale (log (J)–V) and the external quantum efficiency is plottedversus the logarithm of the current density (ηext–log (J)). The representation as afunction of current density instead of the current allows to compare the curves for thedifferent device sizes.

Transmission Line Method (TLM)

The I-V relation is:

contact resistance –EE 432/532 4

In looking at different methods of making contacts, we would like to have standard quantity as point of comparison. The contact resistance depends on the size of the contact, so it is not a good point of comparison. Instead, we can use the contact resistivity.

∆x

Consider a small region in the vicinity of the contact.

So the contact resistivity would have units of Ω·m2. Typical values would range from 10–3 to 10–8 Ω·cm2

Contact resistivity

where AC is the area of the contact.

5& = ք�ӹ[$&

ք& = limӹ[��

(ք�ӹ[) = 5&$&

contact resistance –EE 432/532 5

Using the geometry shown, finding contact resistivity should be a straight-forward exercise in measuring resistors of several lengths and then extracting the parameters, assuming that we know the area of the contacts.

However, we don’t use the contact geometry that was shown. Instead, we have contacts on the top, which is part of the planar geometry.

L

The current flow through the semiconductor is still uniform, but the flow into the contacts is not. Since the current does not flow uniformly in the contact, we can’t use the physical length and width of the contact to determine the contact area.

contact resistance –EE 432/532 6

At the edge of the contact, the current flowing in (or out) is significant. Moving away from that edge, the current drops off until, at the far edge, there is no current.

This is known as “current crowding”.

An analysis of current crowding shows that the drop off in current from the edge of the contact goes as:

(For a more detailed discussion, see chapter 3 of “Semiconductor material and device characterization 2/e” by Dieter Schroder, John Wiley, 1998.)

, ([) � exp

�� [/7

�

contact resistance –EE 432/532 7

LT is the transfer length,

The transfer length is the average distance that an electron (or hole) travels in the semiconductor beneath the contact before it flows up into the contact.

So the effective area of the contact can be treated as LTW.

The contact resistance is then:

LT =

r⇢CRS

57 = 5VHPL + �5&

= 56/: + �

ք&/7:

= 56/: + �56/7:

5& =ք&/7:

=56/7:

57 =56: (/+ �/7)

contact resistance –EE 432/532 8

The plot of RT vs resistor length can also give the transfer length, by extrapolating back to the horizontal axis, where the intercept = –2LT. Thus we know everything needed to find contact resistivity.

There are other techniques for finding contact resistivity. Most of these are discussed in detail in the text from Schroder (reference on slide 6).

RT

L1 L2 L3 L4 L5

L

2RC

slope = RS/W

–2LT

measured resistancecurve fit

57 =56: (/+ �/7)

contact resistance –EE 432/532 9

A typical arrangement for a TLM test pattern is shown below. There is a single rectangular region (blue in the figure) that has the same doping (i.e. same sheet resistance) as the contact areas of the devices. An array of contacts (darker gray in the figure), with various spacings, is formed over the doped region.

W

L1 L2 L3 L4

Resistance measurements between each pair of contacts can be used to construct the TLM graph. From the graph the parameters RS, RC, LT, and ρC can be determined.

contact resistance –EE 432/532 10

ExampleThe TLM patterns for the CyMOS process use, W = 100 µm and L = 10 µm, 20 µm, 40 µm, 80 µm, 160 µm. A set of measurements in the lab for TLM3 (NMOS source/drain diffusion) give the following resistances: R1 = 7.59 !, R2 = 8.26 !, R3 = 9.85 !, R4 = 13.02 !, and R5 = 18.87 !.

5 = �.���ۙ + (Pܟ/ۙ����.�) /

5& =�.���ۙ� = �.���ۙ

56 = VORSH �:

= (Pܟ/ۙ����.�) (Pܟ) = �.��ۙ/

ք& = 5&/7:

= (�.���ۙ) (�.���FP) (�.��FP)

= �.��� ����ۙ · FP�

/7 =�.���ۙ

� (Pܟ/ۙ����.�)= Pܟ�.��

� = �.���ۙ + (Pܟ/ۙ����.�) (��/7)

contact resistance –EE 432/532 11

ExampleA set of measurements in the lab for TLM2 (PMOS source/drain diffusion) give the following resistances: R1 = 76.7 !, R2 = 118.6 !, R3 = 200.8 !, R4 = 362.3 !, and R5 = 648.9 !.

5 = ��.�ۙ + (Pܟ/ۙ���.�) /

RC = 22.6 !.RS = 371 ! / LT = 6.07 µm

ρC = 1.37x10–4 ·cm2

Carrying through the same computations:

What is cathodoluminescence?

Cathodoluminescence (CL) is the emission of light when a material is stimulated by an electron beam.

Cathodoluminescence in a scanning or scanning transmission electron microscope (SEM or STEM) is a unique tool to characterize the composition and optical and electronic properties of materials, then correlate them with morphology, microstructure, composition, and chemistry at the micro- and sub-nanoscale.

Cathodoluminescence microscopy is the analysis of luminescence (emitted light or photons) from a material when stimulated by the electron beam of an electron microscope; the luminescence ranges from ultraviolet to infrared wavelength ranges.

Cathodoluminescence occurs because the impingement of a high energy electron beam elevates the sample to an excited state, which can then induce it to emit a (cathodoluminescence) photon when the sample returns to the ground state. In a semiconductor, this excitation process will result in the promotion of electrons from the valence band into the conduction band, which leaves behind a hole. Therefore when the electron and hole recombine, a photon will emit from the semiconductor.

The photon energy (color) and the probability that a photon (not a phonon) will be emitted depends on the material, its purity, and the defects it contains. Therefore to measure cathodoluminescence, you can examine almost any non-metallic material. In terms of band structure, classical semiconductors, insulators, ceramics, gemstones, minerals, and glasses can be treated the same way. In metals, the excitation of the specimen may be by surface plasmons being launched, which when they decay can result in the emission of a cathodoluminescence photon.

The electron microscope holds distinct advantages over conventional light microscopy, as the spatial resolution of light microscopy is limited by fundamental physics to an approximate resolution of 200 – 300 nm (or half the wavelength of the illuminating source). However in an electron microscope, you can focus the electron beam to a very small spot that potentially gives sub-nanometer spatial resolution. You can also use the emitted signals from the specimen to reveal morphological information, such as size and shape, as well as composition, chemistry, crystallography, electronic properties, plus much more. Thus, the volume of information from a specimen combined with the ability to correlate it directly with information from luminescence (spectroscopy) is what makes cathodoluminescence such a powerful characterization technique.

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USES The characterization of optical properties at extremely small length scales is critical for many areas of scientific research and technologies. Including:

ELECTRON MICROSCOPE SETUP

When the electron beam excites the specimen, this causes luminescence to be emitted from the near-surface region of the specimen. To collect the cathodoluminescence emitted in the upper hemisphere, a mirror is often inserted between the specimen and the pole piece. The mirror is specifically shaped to couple the light out of the microscope vacuum chamber to a spectrograph or photon detector(s). For thin samples, such as electron transparent TEM samples, mirrors above and below the specimen may be employed to collect light emitted in both hemispheres.

When you scan the microscope's focused electron beam in an X,Y pattern then measure the light emitted with the beam at each point, you can obtain an optical activity map of the specimen. The primary advantages of this electron microscope based technique is the ability to resolve features down to 1 nm and correlate the optical properties of an object with structural, compositional and chemical properties measured simultaneously or, at least within the same instrument.

Typical light levels emitted from a specimen can be extremely low and often require you to collect and detect as many photons as possible. Even in samples intended to efficiently emit light, it is important to optimize experimental conditions to collect and detect photons with the minimum of optical losses so you can attain the highest spatial resolution results.