Reporter: 自控三甲 69712007 尤振猛

Bottom cell growth aspects for triple junction

InGaP(In)GaAsGe1043_a三接面 InGaP / (In)GaAs / Ge 底部型太陽能電池成長

指導教授 : 李洋憲

Bottom cell growth aspects for triple junction InGaP/(In)GaAs/Ge solar cells

G. Timò*, C. Flores, and R. Campesato CESI S.p.A. Via Rubattino No. 54 , 20134 Milano, Italy Received 13 December 2004, revised 15 March 2005, accepted 6 April 2005Published online 15 September 2005Key words MOVPE, growth, solar cells, multijunction, GaAs/Ge, AlGaAs/Ge, InGaP/Ge, Ge bottom cell. PACS 81.10.-h

三接面 InGaP / (In)GaAs / Ge 底部型太陽能電池成長

The paper discusses the problems of nucleation layer and substrate specification selection for a bottom Ge cell performance. GaAs/Ge, AlGaAs/Ge and InGaP/Ge heterojuctions have been compared showing how lattice matching and dopant interdiffusion control are key aspects for improving the Ge bottom cell photovoltaic response. The influence of substrates orientation, polarity and resistivity on the electrical performances of the bottom cells are presented.

摘要這篇文章討論成核層和基板規格選擇對底部型鍺太陽能電池性能影響的問題。 GaAs / Ge 、 AlGaAs / Ge 和 InGaP / Ge 的異質接面已經被比較，晶格匹配和控制摻雜物相互擴散是改善鍺底部型電池光電反應速度的重點。基板方向、極性和電阻率對底部電池電性影響也是存在的。

1 IntroductionMulti-junctions (MJ) solar cells made from elements of the groups III-V and IV of the periodic table have been considered by the photovoltaic community for their quasi-ideal energy gap combination which allows the achievement of high solar conversion efficiency [1]. Whereas U.S companies have already showed excellent results producing solar cells reaching AMO efficiency of 29% [2], Europeans suffer of a delay in the development of such a devices and are trying to shorten the technology gap in the frame of European Space Agency programs [3,4].

前言週期表三五和四族元素由於他們類理想的能障結合可達到較高的太陽能轉化效率，已經被考慮製作成多接面太陽能電池 [1] 。美國公司已經展示卓越的太陽能電池生產成果， AMO 的效率可達到 29%[2] ，歐洲人忍受在這些元件發展上的落後並且嘗試在歐洲太空總署計畫架構中縮短技術差距 [3][4] 。

CESI participating these programs, has been working in the realisation of dual and triple junction (TJ) devices for some years by using AIXTRON Planetary rectors and VEECO vertical reactor technologies (see figure 1), facing the related problematic growth aspects and potentiality of the MOCVD systems. We found, for example, that the control of temperature and thickness uniformity has to be carefully taken into account to grow high effciency TJs and a such control is indeed possible even on these production MOCVD systems, in spite of the big size of their reactor cambers.

CESI 參與了這些計畫，若干年後在面臨 MOCVD 系統成長不確定性和潛力下已經利用 Aixtron 和 Veeco 的機台 ( 如圖一 ) 製作出雙與三接面元件 (TJ) 。我們發現小心的考慮控制溫度和厚度均勻性可成長高效率三接面元件，這樣的控制確實可行甚至可在 MOCVD 機台量產，儘管機台大尺寸有隆起問題。

A typical N on P TJ structure, shown in figure 2, is made of several layers, which are deposited to form three junctions (“bottom”, “middle” and the “top” cell) interconnected by two tunnel diodes. The choice of the solar cell materials is the result of a compromise between the demand of optimising light collection and the need of using materials with similar lattice constants and thermal expansion coefficients. A good compromise is accomplished by selecting Ge, (In) GaAs and InGaP semiconductor material for the bottom, middle and top cell, respectively.

典型 N 或 P 三接面結構 ( 如圖二 ) 是由數層組合而成，藉由兩個通道二極體形成互相連接三接面 ( 底部、中間和頂部電池 ) 。太陽能電池的材料選擇要在最佳化光收集和材料相似晶格常數熱膨脹係數中做折衷。一個好折衷方式藉由選擇 Ge 、 (In)GaAs 和 InGaP 分別做為底部、中間和頂部材料被完成。

Ge bottom cell, however, has to be optimised by selecting a proper “nucleation layer”, that is a suitable buffer material to be grown between the substrate and the active region of the device. The nucleation layer plays a fundamental role in the formation of the bottom cell junction and it has the function of passivating its surface [5]. In this paper we focalise our attention on the growth GaAs, AlGaAs and InGaP semiconductors, grown by different reactors, as nucleation layer candidates for bottom Ge cells. The influence of substrate orientation, polarity and resistivity on the electrical performances of the bottom cells will be showed as well.

鍺底部電池藉由選擇合適的成核層已經被最佳化，成核層是可成長在基板與元件主動層中間的一種合適緩衝材料。成核層扮演形成底部電池接面的基本角色，並且具保護表面的功能 [5] 。在這篇文章中，我們集中注意力在不同的機台成長 GaAs 、 AlGaAs 和 InGaP等半導體材料，並當成底部鍺電池的成核層候補。基板方向、極性和電阻率對底部電池電性影響也會被討論。

2 ExperimentGrowth of MJ structures has been carried out by using a Planetary reactor AIX2400 and a vertical VEECO 450 gold reactor. TMGa, TMIn, TMAl, DMZn, AsH3, PH3 and Si2H6 have been used as precursors. Growth pressure in the AIXTRON and VEECO system was changed between 50 and 150 torr. Growth temperature was investigated in the range 650-750°C. Growth rate of GaAs and AlGaAs have been varied between 2 and 10μm/h, while for InGaP, it was maintained fixed around 1.8 μm/h. The solar cell samples were grown on Ge substrates 150 μm thick, with disorientation of 6°, 9° off from [100] to the nearest [111]. The surfacemorphology of the samples was characterised by optical microscopy while in-depth doping profiles were measured by a WEP Electrochemical CV-profiler CVP 21. Photovoltaic performance was measured by using a single source WACOM solar simulator.

實驗藉由使用 AIX2400 與 VEECO 450 成長多接面結構。 TMGa 、 TMIn 、 TMAl 、DMZn 、 AsH3 、 PH3 、 Si2H6 被使用做為前趨物。成長壓力在 AIXTRON 和VEECO 系統都是在 50~150 torr 。成長溫度介於 650~750 度。 GaAs 與 AlGaAs 的長晶速率在 2~10 um/hr之間， InGaP 的長晶速率維持在 1.8 um/hr 。此太陽能電池樣本是成長在 6 度、 9 度且 150 um 厚的鍺基板上。樣本的表面型態利用光學顯微鏡做分析。摻雜分佈利用 WEP electrochemical CV-profiler CVP21 量測。光電特性使用 single source WACOM solar simulator 做量測。

Fig. 1 (a) View of the AIXTRON 2400growth chamber. (b) View of the VEECOGOLD 450 Reactor.

(a)

(b)

圖一 AIXTRON2400 長晶機台外觀 圖二 Veeco450 機台外觀

Fig. 2 Triple junction InGaP/(In)GaAs/Ge solar cell structurewith N on P polarity.

圖二 三接面 InGaP/(In)GaAs/Ge N on P 太陽能電池結構

3 ResultsThe Ge junction can be obtained by atoms solid state diffusion from the nucleation layer to the Ge substrate.Depending on the polarity of the TJ device different solutions can be considered. We have analysed GaAs/Ge, AlGaAs/Ge and InGaP/Ge heterojunctions (HJs), introducing Zn, P, and As as dopants in order to compare the resulting growth morphologies and the electrical performances of the Ge bottom cells. In figure 3 the main results obtained on surface morpologies for the different HJs are shown. GaAs/Ge HJ presents misfit dislocation (MDs) due to the different lattice and thermal expansion coefficient of the two semiconductors, while the growth of lattice matched ternary materials on Ge can be complicated by roughness problem related to the substrate polarity and preparation. In the case of AlGaAs/Ge and InGaP/Ge HJs we found in fact that the amount of arsine introduced in the reactor before growing and the exposure time of the substrate to this atmosphere is crucial for preparing the Ge surface and then improving the morphology. Further the substrate preparation condition are dependent on the polarity of the substrate.

結果鍺接面可利用成核層到鍺基板間進行原子固態擴散獲得。根據三接面元件極性不同方式被考慮。我們分析 GaAs/Ge 、 AlGaAs/Ge 和 InGaP/Ge 異質接面摻入Zn 、 P 與 As為了比較長晶的表面型態和底部型鍺電池的電性。在圖三展示不同異質接面的表面型態。由於兩半導體材料具不同晶格與熱膨脹係數， GaAs/Ge 異質接面存在不合適差排，同時在鍺基板成長第三晶格匹配材料，粗糙度問題相對於基板極性與處理被複雜化。在 AlGaAs/Ge 和 InGaP/Ge 異質接面中，事實上在長晶前從反應爐導入的 As 與基板在大氣暴露時間對於準備表面與改善表面型態是重要的。基板的準備狀況是依據基板的極性。

In the case of GaAs/Ge HJ, we found in the past that the open circuit voltage of the cells is related to the MDs density [see again 5]. In this paper, we report how the GaAs(N)/Ge(N) HJ photovoltaic behaviour is also a function of the temperature profile over the wafer. In the AIX2400 reactor, by regulating the current supply of the infrared lamps installed underneath the susceptor, we could select two different temperature profiles over the wafer and check the photovoltaic behaviour of a GaAs single junction solar cell structure on GaAs/Ge HJ. In particular we measured the open circuit voltage (Voc) distribution, sectioning out of each wafer eigh (2x4 cm2) solar cells. The experiment was performed selecting the “temperature growth window” over which the photovoltage produced by the GaAs/Ge HJ is stronger [6]. The results are reported in figure 4.

在 GaAs/Ge 異質接面，我們發現電池開路電壓與不適合差排密度有關。在這篇文章中，我們報導 GaAs(N)/Ge(N) 異質接面的光電行為是晶圓溫度分佈的函數。在 AIX2400 反應爐，藉由調整 susceptor 底下供應電流的紅外線燈管，我們選擇兩種不同晶圓溫度分佈和確認在 GaAs/Ge 異質接面上成長 GaAs單接面太陽能電池光電行為。我們特別量測每片晶圓(2x4 cm^2) 的開路電壓分佈。這實驗藉由選擇溫度成長範圍使 GaAs/Ge異質接面產生較大的光電壓 [6] 。結果報導在圖四。

Fig. 3 Optical microscope morphology of GaAs/Ge, AlGaAs/Ge and InGaP/Ge HJs. The improvement of surface morphology owing to different substrate preparation is showed. Growths were performed on VEECO 450 reactor. See text for details.

圖三 GaAs/Ge, AlGaAs/Ge and InGaP/Ge 異質接面的光學顯微鏡表面型態。由於不同基板前處理造成表面型態的改善。長晶是在Veeco450 機台完成

Fig. 4 Effect of the growth temperature and temperature profile over the wafer on the open circuit voltage value of GaAs single junction solar cell structure grown on GaAs/Ge HJ. a) Average Voc measured from the 8 cells out of each wafers in function of the growth temperature; b) Temperature distribution over the wafer obtained with a different infrared lamps setting; c) and d) Voc distribution obtained with the different temperature profile; e) solar cell structure.

晶圓上的溫度與溫度分佈對 GaAs單接面太陽能電池開路電壓的影響 (a) 成長溫度改變下的平均開路電壓 (每個晶圓取八個點 ) (b) 不同紅外線燈管設定對應晶圓的溫度分佈 (c)(d) 不同溫度分佈下的開路電壓分佈 (e) 太陽能電池結構

The different Voc values obtained by using the two temperature profiles agree with the thermal distribution over the wafer: higher voltages are found with the temperature profile No.8 which presents higher temperature values all over the wafer. It is possible to conclude that for GaAs (N)/Ge (N) HJs, both temperature and MDs distribution over the wafer influence the uniformity of the photovoltaic performances. Similar results have been obtained by growing the GaAs/Ge HJs with the Veeco reactor, at different growth pressures. We have just to point out that the temperature window over which the Ge cell is more active can be shifted to lower or higher temperatures depending on the reactor type. Since, even with uniform temperature profile the presence of uniformly distributed MDs would limit the GaAs/Ge HJ photovoltage, it would be useful, to consider lattice matched material on Ge substrates, in order to reduce the recombination velocity at the Ge surface. In the case of MJ with P on N polarity we have proposed a new AlGaAs/Ge lattice matched HJ [7], while for N on P devices the InGaP/Ge HJ seems the preferred one.

兩種不同溫度分佈對應的開路電壓與晶圓上的熱分佈一致：當溫度分佈在 NO.8時晶圓有較高的溫度，此時可得到較高的開路電壓。可以推斷 GaAs(N)/Ge(N)異質接面，在兩種溫度和不適合差排分佈影響了光電壓的均勻性。藉由 Veeco機台在不同壓力下長 GaAs/Ge 異質接面可獲得相似的結果。我們證實不同溫度範圍使鍺電池效能更佳，不同機台溫度範圍會降低或升高。均勻的溫度分佈存在均勻的不適合差排將限制 GaAs/Ge 異質接面的光電壓，考慮使用和鍺基板晶格匹配的材料是有益的，可減少在鍺基板界面的再結合速度。我們提出在 P on N多接面上新的 AlGaAs/Ge 晶格匹配的異質接面，同時 N on P 的 InGaP/Ge 異質接面元件似乎更適合此方式 [7] 。

Infact, even if both AlGaAs and InGaP can be grown lattice matched on Ge, AlGaAs presents the advantage with respect to InGaP to allow a better composition control and it is less sensitive to temperature variation. Thus, for the P on N polarity, AlGaAs should be used, while for the N on P polarity, an analysis of the atoms interdiffusion confirms that InGaP is a more suitable ternary material than AlGaAs. We have carried out an easy study to show the strong differences in the diffusion profile among Zn, As, and P which are P-type and N-type dopants in Ge [8].

即使 AlGaAs 和 InGaP 兩者可晶格匹配的成長在鍺上， AlGaAs 和 InGaP 相比具有更佳的組成控制與對溫度變化較不敏感。因此對於 P on N 應該使用 AlGaAs ，同時對於 N on P ，原子相互擴散分析證實 InGa更適合當第三材料。我們已經完成簡單實驗去顯示在 Zn 、 As 、 P 在 Ge 中的擴散分佈有很大的不同 [8]

The simulation of the diffusion profile has been performed by utilising the “thin-film, constant-surface concentration solution” of the Fick’s second law [9], utilising as concentration value at the Ge surface, the solid solubility values of As and P, and in the case of Zn diffusion, the doping level introduced in the ternary layer grown on Ge. The diffusion coefficient was collected from S.M.Sze [10], whereas the solid solubility values were taken from F.A.Trumbore [11]. The calculated diffusion profiles at the growth temperature of 700 °C are reported in figure 5.

藉由使用”薄膜””固定表面濃度”的 Fick’s第二定律可模擬擴散分佈 [9] 。利用 Ge 表面濃度、 As 和 P 的固態溶解度和 Zn 的擴散， Ge 上第三層的摻雜高度可被推導出來。擴散係數已經被 S.M.Sze 收集 [10] ，固態溶解度來自 F.A.Trumbore[11] 。在七百度的擴散分佈計算結果如圖五。

圖五 Zn 、 P 、 As 在 Ge 中的計算擴散分佈 圖六 異質接面結構的極性子分佈

Since the bottom Ge cell efficiency decreases as the thickness of the emitter increases, mainly owing to the lowering of the short circuit current [12], we expect to find a stronger degradation of the bottom cell performances when faster diffuser are utilised, that is, when arsenic is used instead of phosphorous for N on P polarity. Since Zn is the slowest dopant among those considered, we can also conclude that the P on N polarity HJ should be more stable than N on P one. The Ge diffusion into the nucleation layer has to be considered as well. If, for example, InGaP(P)/Ge(N,P) or AlGaAs(P)/Ge(P,N) HJs are grown, high Zn doping levels have to be used in the ternary layers in order to avoid any degradation of the device efficiency. The reason of worsening of the HJ electrical performances is the polarity inversion which can occur in the ternary layer, owing to Ge autodoping from the substrate (see figure 6). The influence of substrate orientation and resistivity is showed for the selected InGaP (N)/Ge(P) and AlGaAs (P)/Ge (N) HJs in table 1.

因為底部鍺電池的效率隨著發光體增加而降低，主要來自於短路電流 [12] ，當更快速的擴散被利用時，我們預期發現更強的底部電池性能劣化，當 N on P 的P 被 As取代時。因為 Zn 是考慮的摻雜物中最慢的，我們也可以推斷 P on N 應該比 N on P 異質接面更穩定。鍺擴散進入成核層同時也被考慮。例如 InGaP(P)/Ge(N,P) 或 AlGaAs(P)/Ge(P,N) 異質接面被成長，高濃度 Zn 摻雜被應用當作第三層為了避免任何元件效能的劣化。異質接面電性惡化的最大原因是由於鍺從基板產生自動摻雜導致第三層極性反轉 ( 如圖六 ) 。基板位向與電阻率對 InGaP(N)/Ge(P) 與 AlGaAs(P)/Ge(N) 異質接面的影響展示在表一。

表格一 不同基板位向與電阻率下未鍍膜異質接面電性總整理

The higher short circuit current values found in the InGaP/Ge HJs with respect to the AlGaAs/Ge HJ are probably due to the better interface between the P-base ternary material and Ge; however, these values are reduced when the remaining materials of the TJ structure are grown, because they have lower energy gap and produce strong light absorption. The following figures has to be considered: InGaP/Ge Isc is 3% reduced when the AlGaAs/GaAs tunnel diode is subsequently grown, and decreased by 19% when another InGaAs layer 1 μm thick is added. When high resistivity substrates are used, the built-in voltage of the Ge junction is lower, consequently the Voc is reduced. High resistivity substrates give rise better results in the case of P on N polarity with respect to the N on P because of the higher value of the lifetime in N-type material. Finally higher substrate orientation improves the surface morphology, the quality of the InGaP/Ge interface, lowering the surface recombination velocity of the Ge sub-cell, and in turn, raising the Voc value.

InGaP/Ge 相對於 AlGaAs/Ge 有更高的短路電流可能是由於較佳的 P 基礎第三材料和鍺界面。然而當三重接面結構的殘存材料成長使數值降低，因為較低的能障和產生較強的光吸收。之後的圖必須被考慮： InGaP/Ge Isc 是 3% ，這個數值減少當 AlGaAs/GaAs 通道二極體接續成長，當另一個 1um 厚的 InGaAs 被成長會減 19% 。當使用高電阻率的基板，鍺接面內建電壓會降低同時開路電壓會減小。高電阻率的基板在 P on N會比 N or P 上具有更佳的效果，因為 N 型材料具有較高的壽命。最後較高的基板位向會改善表面型態、 InGaP/Ge界面、降低鍺基板 - 電池表面再結合速度、增加開路電壓值。

3 ConclusionLattice matching and dopant interdiffusion control along with substrate resistivity and polarity are key aspects in selecting a proper HJ for improving the Ge bottom cell photovoltaic response. Theoretical analisis of dopants diffussion shows that P on N HJs should degrades less than N on P ones when subjected to the thermal load produced by the subsequent MJ cell structure growth. However since as-grown InGaP (N)/Ge (P) HJs shows better short circuit currents than AlGaAs (P)/Ge (N) ones the two selected HJs should shows similar photovoltaic performances after the entire growth cycle of the triple junction cell.

結論除了基板電阻率與極性外，晶格匹配與摻雜相互擴散控制是改善鍺底部電池光電反應選擇合適異質接面的關鍵。當多接面成長時造成溫度負載，摻雜物擴散的理論分析顯示 P on N 異質接面應該比 N on P劣化更慢。然而 InGaP(N)/Ge(P) 比 AlGaAs(P)/Ge(N) 異質接面顯示更佳的短路電流。這兩種選擇的異質接面在完全成長三接面電池後顯示相似的光電性質。

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