Yungryel Ryu

Properties of arsenic-doped p-type ZnO grown by hybrid beam deposition

As-doped ZnO (ZnO:As) films have been characterized. ZnO:As films show p-type characteristics determined by Hall-effect and photoluminescence (PL) measurements. The hole concentration can be increased up to the mid-1017-cm−3 range. The thermal binding energy of the As acceptor (EAth-b) is 120±10 meV, as derived from temperature-dependent Hall-effect measurements. The PL spectra reveal two different acceptor levels (EAopt-b), located at 115 and 164 meV, respectively, above the maximum of the ZnO valence band, and also show the binding energy of the exciton to the As-acceptor (EAXb) is about 12 meV. The values of the ratio EAXb/(EAth-b or EAopt-b) are located in the range from 0.07 to 0.11.

Next generation of oxide photonic devices: ZnO-based ultraviolet light emitting diodes

Results are presented for ZnO-based ultraviolet light emitting diodes (LEDs) that employ a BeZnO∕ZnO active layer comprised of seven quantum wells. Arsenic and gallium are used for p-type and n-type layers. The ZnO-based LEDs show two dominant electroluminescence peaks located in the ultraviolet spectral region between 360 and 390nm, as well as a broad peak at 550nm.

Wide-band gap oxide alloy: BeZnO

A wide-band gap oxide alloy, BeZnO, is proposed and studied in this letter. The BeZnO films were deposited on sapphire substrates by our hybrid beam deposition growth method. The value of the energy band gap of BeZnO can be efficiently engineered to vary from the ZnO band gap (3.4 eV) to that of BeO (10.6 eV). BeZnO can be used for fabricating films and heterostructures of ZnO-based electronic and photonic devices and for other applications. Changes in the measured energy band gap and lattice constant values with Be content are described for BeZnO alloys.

Excitonic ultraviolet lasing in ZnO-based light emitting devices

The authors have fabricated ultraviolet (UV) laser diodes based on ZnO∕BeZnO films. The devices have p-n heterojunction structures with a multiple quantum well (MQW) active layer sandwiched between guide-confinement layers. The MQW active layer comprises undoped ZnO and BeZnO, while the two guide-confinement layers were As-doped p-type ZnO∕BeZnO and Ga-doped n-type BeZnO∕ZnO films, respectively. The exciton binding energy in the MQW region is exceptionally large (263meV). Exciton-related lasing was observed by optically pumping the MQWs. ZnO∕BeZnO-based diodes showed laser action by current injection at room temperature. The lasing mechanism is inelastic exciton-exciton collision.

ZnO devices: Photodiodes and p-type field-effect transistors

The potential use of ZnO-based photonic and electronic devices has been demonstrated by the fabrication of prototype ultraviolet (UV) photodetector and field-effect transistor (FET) devices that contain films of p-type ZnO with arsenic as the p-type dopant. These p-type films have high crystalline quality and show long-term stability. The ZnO UV photodetectors are based on p-n junctions. The FETs are made with metal-semiconductor Schottky contacts on p-type ZnO and are normally off (enhancement) devices. The spectral and electrical characteristics of these devices are presented and explained.

Fabrication of homostructural ZnO p–n junctions and ohmic contacts to arsenic-doped p-type ZnO

We report fabrication of homostructural ZnO p–n junctions that contain arsenic (As)-doped ZnO (ZnO:As) and intrinsic n-type ZnO layers. We also describe the metallization process for forming ohmic contacts to p-type ZnO. ZnO films were synthesized on n-type SiC substrates by hybrid beam deposition. Ni/Au metal contacts show linear I–V characteristics indicative of ohmic behavior, while other metal contacts (e.g., In/Au and Ti/Au) show nonlinear characteristics with rectification that reveal the presence of Schottky barriers. The characteristics for p–n junctions composed of ZnO layers are confirmed by I–V measurements.

Output power enhancement of GaN light emitting diodes with p-type ZnO hole injection layer

We report an enhancement of the optical output power of GaN light emitting diodes (LEDs) by addition of a p-type ZnO layer located in close proximity to the active layer (ZnO/GaN LEDs). Arsenic (As)-doped p-ZnO was used as a hole-injecting layer to overcome the drop in external quantum efficiency of GaN LEDs at high drive currents—the so-called “efficiency droop.” The output power in ZnO/GaN LEDs was improved up to 40%. This result is useful for development of highly efficient GaN LEDs operating at high current densities that will play a critical role in replacement of incandescent lamps by high efficiency solid-state light bulbs.

Characterization of ZnO UV photoconductors on the 6H-SiC substrate

Photoconductors based on wide band gap semiconductors are potential devices for UV light detection due to internal photoelectrical gain and fabrication simplicity. Photoresponses of photoconductors based on GaN and ZnO show high values in UV range under large biases and relatively low values in visible range. Although photoresponse of ZnO photoconductors is similar to that of GaN-based photoconductors, mechanisms of photoconductance between two materials are very different. This difference can be found in optical power dependence of photocurrent and I-V characteristics, and has an impact on device design. In this paper we report experimental studies of photoresponse for newly developed ZnO photoconductors. The ZnO film was grown on a 6H-SiC substrate by hybrid beam deposition. The photoconductor device is formed with interdigitated finger-shaped Ti/Au ohmic contacts on the ZnO film. Electrical characteristics, spectral photoresponse, and persistence properties were studied for the device under variable biases. We find that there are at least three mechanisms involved in the device. At low biases and low incident light power, the photoresponse is mainly due to photocreation. At higher light power and lower biases, the space charge regions are responsible for the photocurrent. At higher biases, the contribution from surface states is dominant.

Optical properties of metal-semiconductor-metal ZnO UV photodetectors

Among wide bandgap materials that are sensitive to photons in the ultraviolet (UV) region, ZnO is a promising photonic material because of its unique optoelectronic properties. Based on the lateral interdigitated back-to-back Schottky contact structure on ZnO film, metal-semiconductor-metal (MSM) photodetectors have substantially lower parasitic capacitance compared with vertical p-i-n photodetectors, which leads to a very high speed photodetection. In this paper, we report optical characteristics of MSM ZnO UV photodetectors for which ZnO films were fabricated by hybrid beam deposition. An annealing process was used in oxygen ambient. The MSM ZnO photodetector consists of two interdigitated electrodes both with Ti/Au metals on an n-type ZnO thin film. The electrodes on the photodetector are finger-shaped. We found that the annealing process decreases contact resistance and photoresponse time. The possible mechanism of annealing process is the removal of surface defects created in the fabrication process. A sublinear power dependence of photocurrent reveals the existence of a light induced space charge region inside the ZnO film. The device displays fast pulse response with a very short rise time and a relatively long relaxation time with applied bias. The exponential decay tail indicates an RC type time response.

Metal‐Semiconductor‐Metal Photodiode Ultraviolet Detector Based on High Quality ZnO

We present mechanism studies of a new metal‐semiconductor‐metal photodiode ultraviolet detector based on high quality ZnO. The detector structure consists of fringe‐type Schottky contacts between high quality n‐type ZnO and Ti or Au metals. High photoresponsivity has been observed at wavelengths above the band edge, which is explained by the generation and trapping of holes near the surface of ZnO. High rejection in the visible makes the device a potential candidate for visible‐blind UV detectors.