Capacitance characteristics with voltage and frequency of n+-GaN/AlxGa1−xN heterojunction ultraviolet (UV)-infrared (IR) photodetectors are reported. A distinct capacitance step and capacitance hysteresis have been attributed to trap energy states located just above the Fermi level at the GaN/AlGaN interface, most likely due to N-vacancy and/or C-donor impurities. The presence of the hysteresis is due to the accumulation of charge at the heterointerface, which is dependent on the location of the continuum of interface trap states relative to the Fermi level. The Al fraction in the barrier layer has been found to significantly change the positions of the interface trap states relative to the Fermi level.

A study of trap states in n +-GaN / AlGaN heterostructures using electrical, thermal, and optical analyses is reported. Capacitance-voltage-frequency measurements showed negative capacitance and dispersion, indicating interface trap states. Infrared spectra identified three impurity related absorption centers attributed to shallow Si-donor ͑pinned to the AlGaN barrier͒, N-vacancy/ C-donor, and deep Si-donor ͑pinned to the GaN emitter͒ impurities with corresponding activation energies of 30.8Ϯ 0.2, 125Ϯ 1, and 140Ϯ 2 meV, respectively. The shallow Si-donor impurity had a relaxation time of 155Ϯ 9 s, while the C-donor/N-vacancy and deep Si-donor impurities appear to behave as a single trap state with a relaxation time of 1.77Ϯ 0.05 s. Multiple analysis techniques allowed the determination of the activation energies of these impurity related centers and the study of the effects of trap states on the electrical behavior of the detector.

Capacitance-voltage-frequency measurements on n + -GaN/Al x Ga 1−x N Heterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors were used to analyze the effects of Al fraction induced heterojunction barrier and its effect on the electrical characteristics at the heterointerface. The detector's IR threshold can be modified by changing the barrier Al concentration. A sample with an Al fraction of 0.1 shows a distinct capacitance step and capacitance hysteresis, which is attributed to N-vacancies and/or C-donor electron trap states located just above the Fermi level (200 meV) at the GaN/AlGaN interface, with activation energies of 149±1 and ∼189 meV, respectively. A sample with an Al fraction of 0.026 showed negative capacitance and dispersion, indicating interface electron trap states located below the Fermi level (88 meV), most likely due to C-donor and/or N-vacancy with activation energies of 125±1 and 140±2 meV, respectively. Additional impurity related absorption centers were identified in both samples, however these shallow Si-donor sites (∼30.9±0.2 meV) did not affect the capacitance as these states were located in the barrier layer and not in the vicinity of the Fermi level. The Al fraction in the barrier layer was found to significantly change the positions of the interface trap states relative to the Fermi level, resulting in the observed capacitance characteristics.

Detection of both UV and IR radiation is useful for numerous applications such as firefighting and military sensing. At present, UV and IR dual wavelength band detection requires separate detector elements. Here results are presented for a GaN/AlGaN single detector element capable of measuring both UV and IR response. The initial detector used to prove the dualband concept consists of an undoped AlGaN barrier layer between two highly doped GaN emitter/contact layers. The UV response is due to interband absorption in the AlGaN barrier region producing electron-hole pairs which are then swept out of the barrier by an applied electric field and collected at the contacts. The IR response is due to free carrier absorption in the emitters and internal photoemission over the work function at the emitter barrier interface, followed by collection at the opposite contact. The UV threshold for the initial detector was 360 nm while the IR response was in the 8-14 micron range. Optimization of the detector to improve response in both spectral ranges will be discussed. Designs capable of distinguishing the simultaneously measured UV and IR by using three contacts and separate IR and UV active regions will be presented. The same approach can be used with other material combinations to cover additional wavelength ranges, e.g. GaAs/AlGaAs NIR-FIR dual band detectors.