Charge Profiling of the p-algan Electron Blocking Layer in AlGaInN Light Emitting Diode Structures

Transcription

1 Mate. Res. Soc. Symp. Poc. Vol Mateials Reseach Society 0892-FF Chage Pofiling of the p-algan Electon Blocking Laye in AlGaInN Light Emitting Diode Stuctues Y. Xia 1,2, Y. Li 1,2, W. Zhao 1,2, M. Zhu 1,2, T. Detchpohm 1,2, E.F. Schubet 1,3, and C. Wetzel 1,2 1 Futue Chips Constellation, Rensselae Polytechnic Institute, Toy, NY 12180, U.S.A. 2 Depatment of Physics, Applied Physics, and Astonomy, Rensselae Polytechnic Institute, Toy, NY 12180, U.S.A. 3 Depatment of Electical, Compute, and Systems Engineeing, Rensselae Polytechnic Institute, Toy, NY 12180, U.S.A. ABSTRACT Chaacteization of opeational AlGaInN heteostuctue light emitting diodes (LEDs) is citical to thei pefomance optimization and time-to-failue analysis. Typically, device pefomance data needs to be cooboated with stuctual infomation such as laye thicknesses, chage pofiles, and the absolute location of the pn-junction. Hee, non-destuctive testing by capacitance-voltage pofiling is being applied to AlGaInN LED stuctues. Within a lage set of samples with diffeent active laye geomety, we obseve distinct layes of high mobile chage accumulation. We coelate those with laye thicknesses deived fom an x-ay diffaction analysis of the coesponding epiwafes. In this way, we identify the chage maxima as the uppe and lowe intefaces of the p-type AlGaN electon blocking laye to the neighboing GaN layes. By means of this successful analysis, we now have the oppotunity to monito epi pocess pefomance and stability as well as device degadation pogess quasi-continuously ove the device lifetime in a non-destuctive mode. INTRODUCTION Goup III nitide light emitting diodes (LEDs) ae the pime candidate fo enegy efficient all solid state lighting in the blue and geen spectal egion. Active egions compising GaN/GaInN multi quantum well (MQW) heteostuctues act as light emittes, while adjacent GaN and AlGaN layes fom a pn-junction to inject electons and holes, espectively. A citical poblem is the contollably and stable injection of eithe caies unde vaious opeating conditions. A p-type AlGaN electon blocking laye seves to educe an electon oveshoot into the p-layes. Placement, doping, and dimensions theeof ae deemed citical to the stabilization of the injection conditions at vaiable dive cuent densities. Capacitance-voltage (C-V) measuements have widely been used to investigate such popeties in, among othes, the AlGaAs/GaAs, InGaAlP/GaAs and ZnO/GaN heteostuctue systems.[1-3] Vey little, howeve, is known fo LEDs in the AlGaInN system. Recently we succeeded in esolving in individual quantum wells GaN/GaInN active layes.[4] Electic pulse and pobe spectoscopy in time domain of fowad biased LEDs has been applied by the Sandia goup.[5] Hee we epot an analysis of the p-type AlGaN electon blocking laye in LED-type stuctues simila to those of blue and geen GaN/GaInN MQW LEDs. We coelate C-V data with esults of x-ay diffaction (XRD) and find good ageement.

2 0892-FF EXPERIMENTAL LED-type AlGaInN heteostuctues have been pepaed by metal oganic vapo phase epitaxy (MOVPE) in an Emcoe D-180 SpectaGaN otating disc multiwafe system using timethyl and diethyl adducts of Ga, In, and Al, as well as ammonia. Ga 1-x In x N/GaN MQW stuctues have been embedded in pn-diodes on (0001) sapphie substate. Ga-face gowth was pefomed along the pola c-axis of GaN. Typical design paametes fo the active egion ae as follows. Seveal Ga 1-x In x N QWs of nominal well width 3 nm, sepaated by baies of nominal thickness of 11 nm have been gown at tempeatues above 650 o C. Thee is no intentional doping in the active egion. The n-side compises 4 μm of GaN:Si doped to a fee electon 18 concentation of ~ 3 10 cm -3. A space laye of nm sepaates the n-side and the quantum well egion. Like the baie layes, this laye is not intentionally doped esulting in 16 typical electon concentations of ~ 3 10 cm -3. The p-side compises an Al y Ga 1-y N electon blocking laye with y = and a nominal thickness of nm. This blocking laye is followed by a GaN p-contact laye of nominal thickness of nm. Both, electon blocking laye and p-contact laye ae Mg doped. The Mg doping concentation typically 17 achieves fee hole concentations of cm -3 in sepaately gown thick p-gan layes. Within the set of samples pesented hee, laye paametes have been vaied within the given boundaies. Epi wafes have been chaacteized by x-ay diffaction in (0002) geomety. The diffaction patten has been simulated using a commecial softwae package fo hexagonal cystal lattices. Expeimental laye thickness data fo the LED-type stuctues was deived fom the simulation. The analysis of x-ay specta involves the simultaneous fitting of some 25 paametes. While we have good confidence in the esulting data, eo bas of +/- 10 % ae possible. C-V data may o may not be taken in exactly the same location of the 2-inch wafe. Data on epi wafes have then been pocessed using a (350μm) 2 LED mask set. The active aea of the mesa was slightly below that. Electo-optical and capacitance-voltage chaacteization was then pefomed on fully pocessed and wie bonded dies. Capacitance was measued at oom tempeatue in the dak using a 10 mv AC modulation at 1 MHz. RESULTS AND DISCUSSION Measued capacitance as a function of bias voltage anging fom fowad injection conditions to deep depletion unde evese bias is shown in Fig. 1. The applied bias is swept fom +2 V to -20 V. The measued capacitance is in the ange of 20 pf to 50 pf. The capacitance shows seveal distinct steps. It is easonable to assume that the majo contibution to the capacitance stems fom the depletion of caies nea the actual pn-junction in the device. Teating the capacitance in the model of a plana capacito, the two plates of which ae fomed by the opposite ends of the depletion zone, the capacitance C is given by A C = ε 0 ε (1) d

3 0892-FF Capacitance (pf) Sample 1 Sample 2 Sample 3 Sample Revese Voltage (V) Figue 1 Room tempeatue capacitance-voltage chaacteistics in some geen LED-type stuctues. whee A is the active aea of the junction mesa, ε = 8. 9 is the elative static dielectic constant of GaN. Togethe with chage Q = CdU = NeAdx (voltage U, electon chage e) we obtain the well-known elations fo fee caie density N as a function of depletion width x: A x = ε 0 ε (2) C 3 du C n( x) = (3) 2 dc ε 0ε A e Depletion will occu on both plates of the so-fomed capacito and the depletion width x is the sum of contibutions x a and x b on both ends, x = x a + xb. Accodingly, n (x) is a weighted caie concentation aveaging concentations n a and n b on eithe side. Unde ealistic conditions, one concentation will dominate, e.g., n b >> na, and depletion into egion a will dominate: x xa, n( x) na. In a ealistic stuctue, the chage concentation may vay significantly with depth on both, n- and p-sides of the junction. In paticula, a fou laye system compising n-gan, active egion, p-algan and p-gan layes with a pn-junction at the inteface of active egion and p-algan might hold the following fee caie concentations: n n GaN > p p GaN > p p AlGaN > nactive _ egion. In this case, stating fom fowad injection condition towads evese bias condition, depletion would fist dominate in the active egion. Upon eaching the inteface of n-gan and active egion, depletion would continue with the p-algan laye, which has the next highe density of chages. Afte full depletion of the p-algan laye, pedominant depletion would continue with the p-gan laye. Along this sequence, chage densities of altenating sides of the oiginal pn-junction would be pofiled. The odinate x of the total depletion width theefoe epesents the thickness sum of successively depleted layes with inceasing caie concentations.

4 0892-FF p-algan Caie Concentation (cm -3 ) p-algan Active laye thickness Active laye thickness Sample 1 Sample 2 Sample 3 Sample Depletion Width (nm) Figue 2 Calculated mobile caie concentation vesus depletion width. Intepetation of the expeimental data along equations (1) - (3) is pesented in Fig. 2 fo fou diffeent dies of slightly diffeent stuctue. In all cases, with inceasing convoluted depletion width x, the intepeted caie concentation aises fom values of cm -3 to naow 18 spikes with values of ~ 3 10 cm -3. Fom thee on, the concentation falls to values nea 17 ~ 5 10 cm befoe it eaches anothe high point with values nea ~ 2 10 cm -3. The peak positions on the convoluted depletion width axis fo both maxima fo some 22 samples ae collected and analyzed in the following figues. Fig. 3 coelates the absolute position of the fist peak with the total thickness of the nominally undoped active egion compising wells, baies, and space as deived fom the x-ay diffaction analysis. As an additional function, a line of slope 1 though the oigin is shown. Within the lage expeimental data set, thee is an excellent coelation between both quantities. This suggests that the fist peak in the fee caie concentations maks the completed depletion of the nominally undoped active egion. This egion is suounded by the p-type AlGaN electon blocking laye and the highly n-type GaN contact laye. The expeimental spacing between the fist and the second maximum fo some 25 analyzed samples is shown in Fig. 4 togethe with the thickness of the AlGaN electon blocking laye as detemined fom the x-ay diffaction data. To guide the eye, a line of slope 1 though the oigin is also shown. Within the lage set of data thee is a athe good coelation of both quantities. This stongly suggests that the sepaation of both concentation maxima indeed is the thickness of the AlGaN electon blocking laye. While placing the assignment solely on the data in Fig. 4, would not be vey convincing, the fact that one side of the electon blocking laye lies vey close to the expected sepaation fom the pn-junction is stong supplementay evidence. The fee caie concentation value in the high cm -3 should be an indication of its fee hole concentation. Reliable data beyond the second peak could so fa not be obtained. It theefoe is not possible to identify which laye is being depleted next. This can eithe be the GaN p-contact laye o the GaN n-contact laye. The above association of depleted layes is indicated by labels in Fig. 2.

5 0892-FF Active Region Thickness fom XRD (nm) Position of Fist n Peak (nm) Figue 3 Thickness of the active egion deived fom n(x) vesus that fom XRD. AlGaN Thickness fom XRD (nm) Sepaation of Fist and Second n Peak (nm) Figue 4 Thickness of the AlGaN electon blocking laye deived fom n(x) vesus that fom XRD. Detailed analysis of C-V pofiles acoss heteointefaces has been developed by Koeme et al. [6]. Accoding to thei findings, the fist momentum of the excess fee chage density aound the inteface is a measue of the potential dop at the inteface and can be elated to the elative band offset. An analysis of ou data esults in values within the easonable ange of mev. A moe accuate intepetation in ou case is not possible due to inaccuate knowledge of dopant concentations typical fo goup-iii nitide heteostuctues and the highe complexity of ou stuctues.

6 0892-FF In the simplified case of a GaN/GaInN/GaN double heteostuctue, Zhang et al [7] popose a coelation of the measued fee chage density in the maxima of n(x) with the fixed polaization chage at the heteointeface. In thei case, the locally measued fee chage density n(x) that exceeds the local concentation of ionized dopants should compensate the fixed polaization chages at one of the intefaces. The total excess fee chage unde the maximum theefoe should be a diect measue of the inteface polaization chages. Despite entiely diffeent stuctues, we obtain values of the same ode of cm -2 in ou expeiments. In ou case, these values should coespond to the polaization at the GaN/AlGaN heteointeface between the last baie of the active egion and the electon blocking laye. CONCLUSIONS In summay, we investigated C-V chaacteistics of the caie injection layes of LEDtype AlGaInN MQW stuctues. We popose that deived naow fee caie maxima indicate the step-wise depletion of active egion and the electon blocking laye when the applied voltage is swept fom fowad to evese bias. In this way, we diectly detemine the thickness of the active egion and the pesence and appoximate thickness of the electon blocking laye. The data deived in standad C-V measuement coelates closely with data fom x-ay diffaction measuements and the designed stuctue. Togethe with a pevious detailed analysis of the active egion in LED-type GaInN/GaN heteostuctues, this identification and quantification of the AlGaN electon blocking laye in convenient capacitance voltage chage pofiling povides a most suitable handle fo the fast and eliable LED device chaacteization and optimization. Step-by-step we ae developing the tools to analyze the entie device stuctue and the exact location of the pn-junction. It theefoe should pove suitable fo apid advancement of goup-iii nitide LED technology fo all solid state lighting. ACKNOWLEDGMENTS We thank Lumionics Nitide Consulting fo poviding the epi mateial used in this study. REFERENCES [1] Q. D. Qian, M. R. Melloch, and J. A. Coope, J., Appl. Phys. Lett. 48, 638 (1986). [2] M. O. Watanabe and Y. Ohba, Appl. Phys. Lett. 50, 906 (1987). [3] D. C. Oh, T. Suzuki, J. J. Kim, H. Makino, T. Hanada, T. Yao, and H. J. Ko, Appl. Phys. Lett. 87, (2005). [4] Y. Xia, E. Williams, Y. Pak, I. Yilmaz, J.M. Shah, E.F. Schubet, and C. Wetzel, Mate. Res. Soc. Symp. Poc. Vol. 831 E3.38 (Mateials Reseach Society, Waendale PA, 2005). [5] R. J. Kapla, S. R. Kutz, D. D. Koleske, A. A. Alleman, A. J. Fische, and M. H. Cawfod, Mate. Res. Soc. Symp. Poc. Vol. 831 E (Mateials Reseach Society, Waendale PA, 2005). [6] H. Koeme and Wu-Yi Chien, Appl. Phys. Lett. 36, 295 (1980). [7] H. Zhang, E. J. Mille and E. T. Yu, Appl. Phys. Lett. 84, 4644 (2004).