Ho Ki Kwon

Back illuminated AlGaN solar-blind photodetectors

We report the growth, fabrication, and characterization of AlxGa1−xN (0⩽x⩽0.60) heteroepitaxial back-illuminated solar-blind p-i-n photodiodes on (0001) sapphire substrates. The group III-nitride heteroepitaxial layers are grown by low-pressure metalorganic chemical vapor deposition on double polished sapphire substrates using various growth conditions. The back-illuminated devices exhibit very low dark current densities. Furthermore, they exhibit external quantum efficiencies up to 35% at the peak of the photoresponse (λ∼280 nm). Improvements were made to the growth technique in order to achieve crack-free Al0.4Ga0.6N active regions on a thick Al0.6Ga0.4N window layer and to obtain activated p-type Al0.4Ga0.6N layers.

Temperature dependent common emitter current gain and collector-emitter offset voltage study in AlGaN/GaN heterojunction bipolar transistors

We have demonstrated state-of-the-art performance of AlGaN/GaN heterojunction bipolar transistors (HBTs) with a common emitter (CE) current gain of 31 at 175 K and 11.3 at 295 K. The increase in collector current and CE current gain at lower temperature can be attributed to the reduced base-emitter interface recombination current. We also observed an increase of collector-emitter offset voltage with decrease of temperature. The increase of V/sub CEOFF/ at lower temperature is related to an increase of V/sub BE/ as the base bulk current is increased, or to the reduction of the ideality factor n/sub BE/.

Photoluminescence and recombination mechanisms in GaN/Al0.2Ga0.8N superlattice

A detailed study of photoluminescence (PL) of GaN(1 nm)/Al0.2Ga0.8N(3.3 nm) twenty periods superlattice grown via metal-organic chemical vapor deposition is presented. The dependence of the PL emission energy, linewidth, and intensity on temperature, in the low temperature regime, is consistent with recombination mechanisms involving bandtail states attributed to a small degree of interfacial disorder. The activation energy of the nonradiative centers in our superlattice agrees well with the value we derive for the width of the tail-state distribution. Moreover, we find that the average phonon energy of the phonons that control the interband PL energy at high temperatures is larger for the superlattice than for a high-quality GaN film. This observation is consistent with model calculations predicting the phonon mode properties of GaN–AlN-based wurtzite heterostructures.

Observation of long photoluminescence decay times for high-quality GaN grown by metalorganic chemical vapor deposition

GaN epitaxial layers with different crystalline quality grown on sapphire substrates by metalorganic chemical vapor deposition are investigated using time-resolved photoluminescence at 300 K. It is found that the time-dependent photoluminescence of low-quality GaN decays faster than that of the high-quality GaN films. The time constants for the dual-exponential decay of the photoluminescence are calculated to be 50 and 250 ps for high-quality undoped GaN and 30 ps for low-quality undoped GaN. For high-quality Si-doped GaN, time constants of 150 and 740 ps are extracted while corresponding time constants of 40 and 200 ps are measured for low-quality Si-doped GaN. We believe that the time constant of 740 ps measured for our high-quality Si-doped GaN is the longest ever reported for thin GaN/sapphire films.

Optical properties of undoped and modulation-doped AlGaN/GaN single heterostructures grown by metalorganic chemical vapor deposition

The optical properties of undoped and modulation-doped AlGaN/GaN single heterostructures (SHs) grown by metalorganic chemical vapor deposition are investigated at low temperature using photoluminescence measurements. The formation of a two-dimensional electron gas at the heterojunction is verified by temperature-dependent Hall mobility and 300 K capacitance-voltage measurements. Radiative recombination is observed between the electrons in two-dimensional quantum states at the heterointerface and the holes in the flat-band region or bound to residual acceptors both in undoped and modulation-doped AlGaN/GaN SHs. These peaks disappear when the top AlGaN layer is removed by reactive ion etching. In addition, the photoluminescence results under different laser excitation intensity and lattice temperature are also described for undoped and modulation-doped AlGaN/GaN SHs with various Al compositions and growth interrupt times.

Radiative recombination of two-dimensional electrons in a modulation-doped Al0.37Ga0.63N/GaN single heterostructure

We have studied the low-temperature (4 K) photoluminescence (PL) of a modulation-doped Al0.37Ga0.63N/GaN single heterostructure. Radiative recombination is observed between electrons in the two-dimensional quantum states at the heterointerface and holes in either the flatband region or bound to residual acceptors. This PL emission disappears when the top AlGaN layer is removed by reactive ion etching, indicating that the luminescence is from the two-dimensional electron states. From the peak separation of the luminescence related to the two-dimensional electrons, the maximum electric field at the heterointerface is estimated to be 3.7×104 V/cm.

The growth of AlGaN/GaN heterojunction bipolar transistors by metalorganic chemical vapor deposition

The growth and properties of AlGaN/GaN heterojunction bipolar transistors grown by low-pressure metalorganic chemical vapor deposition (MOCVD) are described. The emitter-up structures are grown on (0001) sapphire substrates. Common-emitter characteristics and Gummel plots were measured on 120 x 120 μm 2 devices and useable DC current gains of β ∼ 8-14 were achieved at room temperature. The resistance of the base was a limiting factor in high-current operation of these devices.

Optical properties of undoped and modulation-doped AlxGa1−xN/GaN heterostructures grown by metalorganic chemical vapor deposition

Abstract The radiative recombination of carriers in a two-dimensional electron gas (2DEG) in undoped and modulation-doped Al x Ga 1− x N/GaN heterostructures grown by metalorganic chemical vapor deposition is investigated at low temperature using photoluminescence measurements. Temperature-dependent Hall effect and Shubnikov–de Haas measurements, verify the formation of a high-quality 2DEG. Radiative recombination is observed between the 2DEG in quantum states at the hetero-interface and the holes in the flat-band region or bound to residual acceptors both in undoped and modulation-doped Al x Ga 1− x N/GaN heterostructures. The luminescence peaks related to the 2DEG disappear when the top AlGaN layer is removed by reactive ion etching. In addition, the effect of the growth interruption time, laser excitation intensity, and doping conditions upon the photoluminescence is also described.