Recent Progress in AlGaN/GaN Based Optoelectronic Devices
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Energy Gap (eV) Figure 1: Energy gaps of wide band gap semiconductor materials with the spectral sensitivity of a human eye.
In this paper, we review our recent results on GaN based optoelectronic devices, which include, InGaN-AIGaN Light Emitting Diodes (LEDs), GaN p-n junction ultraviolet detectors, and optoelectronic AlGaN-GaN Heterostructure Field Effect Transistors. 913 Mat. Res. Soc. Symp. Proc. Vol. 395 ©1996 Materials Research Society
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EPITAXIAL LAYERS
All these devices use the epitaxial layers deposited using low pressure MOCVD over basal As reported elsewhere 1, 2, 3 we use triethylgalium, plane sapphire substrates. triethylaluminum and ammonia as the precursors for 'Ga', 'Al' and 'N'. Typical growth pressure and temperature are 76 torr and 1000 oC. As deposited the GaN layers are highly resistive with a carrier density well below 1015 cm- 3. The insulating GaN layers can be doped either n- or p-type using disilane (Si) or bis-Mg (Mg) as the dopants. The single epilayers and the heterojunctions were characterized for their electrical and optical properties. The two dimensional electron gas at the AlGaN/GaN heterointerface 2 exhibits room temperature mobilities in excess of 1200 cm 2/V-sec and over 5000 cm /V-sec at 80 K. 3,4
III. PHOTOCONDUCTIVE, DETECTORS
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We fabricated GaN based photoconductive and photovoltaic ultraviolet (UV) detectors. 5, 6, 7 As an example, we show in Figure 2 the current-voltage curves for a p-n junction GaN UV detector. The doping level of the p- and the n-type layers in the junction region was around 5x10 6 cm-3. A 1.5 V turn-on and a reverse breakdown in excess of 10 V is observed. Figure 3 shows the responsivity of this detector as a function of wavelength. As seen no photoresponse is observed for wavelengths larger than the band gap (365 nm). Below 365 nm. the responsivity is nearly independent of the wavelength indicating a very low surface recombination. This visible-blind detection feature was also observed for our GaN photoconductive and Schottky barrier detectors. 6,7
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Voltage (V) 2: I-V curve of p-n junction GaN UV Figure detector. 5
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Figure 3: The photoresponsivity of GaN p-n junction versus wavelength. 5
IV. LIGHT EMITTING DIODES Figure 4 we present the light emission from a GaN homojunction for a forward current of 20 mA across a 200x200 micron LED. 8 We have also demonstrated optically pumped stimulated deposited over basal plane sapphire emission from GaN 9 and InGaN '0. These layers were substrates. A threshold power of around 1 MW/cm 2 was measured in each case. Figure 5 shows the vertical cavity photoluminescence signal from a GaN-InGaN heterojunction as a function of wavelength using a pulsed nitrogen laser (337 nm) for pumping. As seen from the
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figure, Fabry-Perot fringes a
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