Low-voltage Avalanche Breakdown in AlGaN Multi-quantum Wells
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Low-voltage Avalanche Breakdown in AlGaN Multi-quantum Wells Shengkun Zhang1, X. Zhou1, Wubao Wang1, R. R. Alfano1, A. M. Dabiran2, A. Osinky2, A. M. Wowchak2, B. Hertog2, C. Plaut2, and P. P. Chow2 1 Physics Department, City College of New York, 160 Convent Avenue, New York, NY, 10031 2 SVT Associates, Inc., Eden Prairie, MN, 55344 ABSTRACT
In this work, electroluminescence (EL) of a AlGaN p-i-n diode have been investigated in both avalanche and injection modes. The active i-region of the diode consists of Al0.1Ga0.9N/Al0.15Ga0.85N MQWs. Strong interband luminescence from the Al0.1Ga0.9N active layers was observed when operating the device in both avalanche and injection modes. The threshold voltage for avalanche breakdown is as low as 9 V. This indicates that the impact ionization coefficient of electrons is greatly enhanced in these Al0.1Ga0.9N/Al0.15Ga0.85N MQWs comparing to AlGaN bulk materials. Polarizationinduced electric fields in the Al0.1Ga0.9N well layers are believed to be responsible for the enhancement of the ionization coefficient. In a control sample that has higher defect density, the electroluminescence was dominated by long-wavelength emissions, which results from impact ionizations of the defect levels. INTRODUCTION
Avalanche breakdown is difficult to be realized with AlGaN materials since extremely high electrical fields are required to accelerate carriers to threshold energy that is much higher than their large bandgaps. For instance, to reach avalanche breakdown in GaN diodes, a typical reverse bias larger than 40 V is required [1-4]. Breakdown electroluminescence (EL) in GaN is normally dominated by defects according to these reports. We report here the successful operation of a AlGaN avalanche diode and the breakdown voltage is as low as 9 V. EXPERIMENTAL DETAILS
The AlGaN LED devices were fabricated over 6H-SiC substrate. The growth sequence is as follows: a thin AlN buffer, 0.3 µm heavily doped n-type Al0.25Ga0.75N layer, 20 nm undoped Al0.3Ga0.7N layer, three period undoped Al0.1Ga0.9N (3 nm) /Al0.15Ga0.85N (5 nm) quantum wells, 20 nm undoped Al0.23Ga0.77N layer, five period heavily doped p-type GaN (10 nm) /Al0.1Ga0.9N (5 nm) quantum wells, 0.2 µm p-type GaN layer and 10 nm heavily doped p-type GaN. The wafer was then cut into square pieces with an area of 1 mm2. Ohmic contacts were made on both front and back surfaces. A control sample that has the identical structure but high density of defects is
used to investigate the effect of defects. Current-voltage (I-V), EL and capacitancevoltage (C-V) measurements were done at room temperature to characterize the fabricated LEDs. In the EL experiments, light emitting out from the front surface was collected as luminescence signal. RESULTS AND DISCUSSION Figure 1 shows the electroluminescence spectra of the diode measured under a reverse bias of 14 V (black line) and a forward bias of 5 V (red line), both producing current of about 20 mA. The EL spectrum of the control sample was also recorded under a reverse current of 20 mA.
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