WOCSDICE 2004, Smolenice Castle, Slovakia, May

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1 WOCSDICE 2004, Smolenice Castle, Slovakia, May

2 WOCSDICE 2004, Smolenice Castle, Slovakia, May

3 WOCSDICE 2004, Smolenice Castle, Slovakia, May

4 WOCSDICE 2004, Smolenice Castle, Slovakia, May Numerical simulation and analysis of GaP/GaN X P 1-X /GaP double heterostructure lightemitting diode L. Peternai 1, J. Kovac 1, J.Jakabovic 1, V. Gottschalch 2 1 Slovak University of Technology Bratislava, Ilkovicova 3, Bratislava, Slovakia 2 University of Leipzig, Linesstrasse 3, Leipzig, Germany Abstract Light-emitting diode including a novel type of ternary alloy GaP/ GaN X P 1-X /GaP structure was numerically analyzed. For such a structure electrical and optical parameters like: volt-ampere characteristic, band diagram, current spreading through the structure, electrical field, spectral characteristic and light ray-trace were simulated and compared with real LED structure. Introduction Solid light sources and displays are currently of great interest for application of lightemitting diodes (LEDs). Recently GaN X P 1-X represents a novel material and has attracted considerable interest for LED fabrication in green-red range of visible spectrum. It is well known that the incorporation of N as a dopant to GaP forms recombination centers called isoelectronic traps, which offer rare green light emission [1]. The development of GaN X P 1-X alloy started few years ago. By using large pseudopotencial supercell calculation was found, that phosphor rich ternary GaN X P 1-X alloys shows bandgap anomalies [2]. The predicted quasi-direct crossover composition was determined at x = This was verified from photoluminescence (PL) emission intensity increasing at room temperature for N concentration up to [3]. For low concentration of N in PL emission sharp lines are detected due to NN i pairs [4]. The interaction of nitrogen pairs in GaN X P 1-X alloys forms a complex series of deep gap levels bellow the conduction band (CB) [5]. Experiment The structure growth was performed using low pressure MOVPE process. As a source trymethilgallium, phosphide and dimethylhydrazine was used. The nitrogen concentration was determined by high resolution X-ray difraction measurement. The double-heterostructure consisted of GaP substrate (n= cm -3 ), 100 nm GaP buffer layer, 130 nm GaP bottom cladding layer (n= cm -3 ), 130 nm GaN X P 1-X active layer and 130 nm top cladding GaP layer (p= cm -3 ). Ohmic contacts were deposited Energy [ev] GaP substrate n - GaP c ladd ing lay er GaN X P 1-X active layer p - GaP cladding layer Distance [µm] Figure 1. Band diagram under forward bias on LED structures and annealed in forming gas atmosphere. To simulate the physical properties of the LEDs the device simulation software APSYS [6] was employed. This software allowed 1D simulation of the heterostructure devices. The simulation of the bandgap change in undoped GaN X P 1-X at room temperature was determined as: Eg = *x *x^2 Advantage of GaP/ GaN X P 1-X /GaP structure is the utilization of wide bandgap GaP substrate properties and optical transparency above wavelength range

5 102 WOCSDICE 2004, Smolenice Castle, Slovakia, May 2004 forward current [A] sample a sample b sample c sample d sample e simulated forward voltage [V] forward current [A] E-3 1E-4 1E-5 1E-6 1E-7 1E-8 sample a sample b sample c sample d sample e simulated 1E forward voltage [V] Figure 2. IV characteristic: straight lines represent measured samples, dash line the simulated characteristic 550nm. For LED simulation the nitrogen concentration 1.5% was chosen. In Fig. 1 depicted the simulated band gap diagram is under the forward bias for GaP/ GaN X P 1-X /GaP structure diode. Corresponding current voltage characteristics are shown in Fig. 2. This include the simulated I-V characteristic (dashed line), and the set of measured samples characteristics. The differences in I- V characteristics are caused primarily by the contact quality and leakage current through dislocations in the real structure. The evaporated ohmic contacts represent a real resistance connected serially to LED structure, while leakage current increase the current flow and reduces the total voltage applied at low current. EL Intensity [r.u.] FWHM = 30nm sim ulated EL emission λ max = 615nm Wavelength [nm] emission from nitrogen levels lying deeper under the conduction band. EL Intensity [r.u.] measured EL emission λ max = 617nm W av elen gth [n m ] FWHM = 64nm Figure 3. Simulated and measured EL spectrum for GaP/GaNP/GaP double heterostructure diode. The calculated EL spectra is depicted in Fig. 3a and measured in Fig. 3b. The measured emission maximum is in good correspondence with the simulated value of 617nm. The difference in the red tail of emission spectra is due to the Conclusion The double heterostructure GaP/ GaN X P 1-X /GaP light-emitting diode was fitted using numerical simulation. Electrical and optical characteristics were extracted and compared with real diode. The simulated characteristic shows good agreement with the measured basic properties of LED. Acknowledgement This work was supported by the VEGA 1/0152/03 grant of Slovak Grant Agency and EC project No. IST VGF GaP LEDs and bilateral cooperation project References [1] A. A. Bergh, P. J. Dean: Light-emitting diodes, Claredon Press Oxford 1976, ISBN [2] L. Bellaiche, S.-H. Wei, A. Zunger: Physical review B Vol. 56, No. 16, 1997, page [3] W. Shan, et. al. Physica Status Solidi (b) Vol. 223, No. 75, 2001, page [4] Y. Zhang, et. al. Physical review B Vol. 62, No. 7, 2000, page [5] P. R. C. Kent, A. Zunger: Physical review B Vol. 64, 2001, page [6] Crosslight Software Inc.