S. Nagar

Influence of Li implantation on the optical and electrical properties of ZnO film

ZnO has been a subject of intense research in the optoelectronics community owing to its wide bandgap (3.3eV) and large exciton binding energy (60meV). However, difficulty in doping it p-type posts a hindrance in fabricating ZnObased devices. In an attempt to make it p-type we have studied Li-implanted (Energy=40keV, dose=5x1013cm-2) <002> ZnO films grown over <001> sapphire substrates by Pulsed Laser Deposition technique at 400°C (sample A). The samples were subsequently subjected to Rapid Thermal Annealing at 650° and 750°C (samples B and C) for 30 seconds. Room temperature Photoluminescence study of as-deposited sample reveal consistent Donor-bound exciton (D0X) peak at 3.3eV, which shifts to 3.298eV, 3.298eV, and 3.289eV for samples A, B and C respectively. This data validates the n-type conductivity of the samples with a carrier concentration and Hall mobility of 8.68x1019cm-3, 1.13x1019cm-3 and 2.9x1020cm-3 and 2.14cm2/V-sec, 35.2cm2/V-sec, 16.9cm2/V-sec for samples A, B and C respectively. The reduced energy of D0X peak is probably due to strain variations occurred during the various processing steps. While the higher carrier concentration in sample C can be attributed to aggregated vacancy clusters at high temperature annealing. Since Li acts as an acceptor for ZnO, so a free electron-acceptor (FA) peak at 3.227eV, 3.217eV and 3.225eV in samples A, B and C is evident. A third peak at 3.128eV may be due to the donor-acceptor pair, a reason for a lower energy FA peak for sample B.

P-type ZnO films by phosphorus doping using plasma immersion ion-implantation technique

ZnO has been a subject of intense research in the optoelectronics community owing to its wide bandgap (3.3eV) and large exciton binding energy (60meV). However, difficulty in doping it p-type posts a hindrance in fabricating ZnO-based devices. In order to make p-type ZnO films, phosphorus implantation, using plasma immersion ion-implantation technique (2kV, 900W, 10μs pulse width) for 30 seconds, was performed on ZnO thin film deposited by RF Magnetron Sputtering (Sample A). The implanted samples were subsequently rapid thermal annealed at 700°C and 1000°C (Samples B and C) in oxygen environment for 30 seconds. Low temperature (8K) photoluminescence spectra reveal dominant donor-bound exciton (D°X) peak at 3.36eV for samples A and B. However, for Sample B the peaks around 3.31eV and 3.22eV corresponding to the free electron-acceptor (FA) and donor to acceptor pair peaks (DAP) are also observed. A dominant peak around 3.35eV, corresponding to acceptor bound exciton (A°X) peak, is detected for Sample C along with the presence of FA and DAP peaks around 3.31eV and 3.22eV. Moreover, the deep level peak around 2.5eV is higher for Sample B which may be due to implantation and acceptor related defects. However, for Sample C, the deep level peaks are very weak compared to the near band edge peaks confirming that these peaks are mainly due to intrinsic defects and not related to acceptors. These results clearly show us a promising way to achieve p-type ZnO films using phosphorus doping.

Enhancement (100 times) of photoluminescence in pulsed laser deposited ZnO thin films by hydrogen ion implantation

Owing to its wide bandgap (3.37eV) and a large exciton binding energy (60meV), fabrication of ZnO based optoelectroincs devices is a very active research area. Hence, enhancing the photoluminescence of the ZnO films will be important to achieve higher efficiency optoelectronic devices. Hydrogen ion implantation (Energy = 50keV, Dose = 5×1012cm-2) have been performed on Pulsed Laser Deposited ZnO thin films deposited at 650°C. The samples were subsequently subjected to Rapid Thermal Annealing at 750°C, 800, 850°C and 900°C for 30 seconds in oxygen environment. X-ray Diffraction study confirms deposition of highly oriented <002> ZnO films for all the samples. However, the peaks for the samples are not at the same position due to the strain induced during implantation and subsequent annealing. Low temperature photoluminescence (8K) spectra of the samples revealed the presence of peaks of donor-bound exciton (D°X) and free-exciton (FX) at 3.36eV and 3.37eV respectively. Deep level defect peak around 2.5eV was also observed in the samples but the intensity of these peaks was substantially weaker than the near band edge (NBE) peaks verifying the high quality of the films. Moreover, the integrated PL peak intensity of the NBE show that the luminescence gets considerably enhanced as the samples are implanted (4 times) and subsequently annealed (up to 100 times) when compared with the as-deposited sample. Thus, implanting hydrogen ions maybe a good way to enhance the photoluminescence and thus efficiency for ZnO based devices.

Optimisation of PLD Parameters

Progress in the fabrication of ZnO-based optoelectronic devices lies in producing reproducible, reliable and stable p-type ZnO films, because of the intrinsic n-type nature of the deposited ZnO films. Hence, for successful conversion of its carriers from n-type to p-type, it is desirable that the deposited ZnO film has as low an electron concentration as possible. Moreover, for the fabrication of optoelectronic devices, the films should have very high optical quality. The growth of highly oriented films with the least strain will offer an added advantage in fabricating these devices. Hence, the deposition parameters of temperature and pressure using pulsed laser deposition (PLD) were optimised keeping these things in mind. A substrate temperature of 650 °C and oxygen pressure of 40 mTorr were found to be optimised growth parameters as it had the lowest carrier concentration of 1.01 × 1017 cm−3 a reasonably high Hall mobility of 16.1 cm2 V−1 s−1 and also had the highest optical quality. Once the PLD parameters were optimised, hydrogen implantation was carried out to see whether it further enhanced the electrical and optical properties of the thin film. While the Van der Pauw Hall measurements did not reveal any significant changes in the electrical characteristics of the thin films, the optical quality of the implanted films was found to increase by two orders of magnitude when compared to the as-deposited sample. Such an enhancement in the optical luminescence of the ZnO thin films may be helpful in fabricating highly efficient ZnO-based devices.

Fabrication of Optoelectronics Devices

Once reliable and reproducible p-type ZnO was achieved, different heterojunction and homojunction devices were fabricated. The heterojunction p–n junction diode was fabricated by depositing p-type ZnO films over low resistivity n-Si substrates. The p-type ZnO was achieved by phosphorus as well as nitrogen implantation. The current–voltage characteristics of the fabricated p–n heterojunction diode showed a clear rectifying behaviour with a threshold voltage of 3.3 V was observed for the phosphorus-implanted device. For the nitrogen-implanted device, a threshold voltage of 1.7 V was attained. This low threshold voltage may help to increase the lifetime of the devices. Homojunction LED was fabricated over high resistivity n-Si substrates, such that the effect of substrate conduction is nullified. The p-type ZnO was deposited by the phosphorus and nitrogen implantation as described above. The n-type ZnO was deposited over the p-ZnO by PLD technique using the optimised temperature and oxygen parameters of 650 °C and 40 mTorr, respectively. Room temperature electroluminescence (RTEL) of the fabricated device (phosphorus implantation) clearly depicted the emission of UV light around 3.18 eV. However, EL emission is dominated by a broad emission around 1.8 eV. This observation is based on the presence of deep-level defects in ZnO films. RTEL for the device fabricated by nitrogen-implanted p-ZnO showed a broad emission around 1.8 eV, corresponding to the red emission, is observed from the device. However, no UV emission was observed from the fabricated device.

Optimization of ZnO Thin Films

Extensive research is currently carried out on ZnO as a potential material for the fabrication of optoelectronic devices such as photodiodes, laser diodes (LDs) and light-emitting diodes (LEDs) in the ultraviolet region because of its wide bandgap (3.437 eV at 2 K) and a large excitonic binding energy of 60 meV at room temperature. However, because of intrinsic defects such as oxygen vacancies and zinc interstitials, ZnO is intrinsically deposited as an n-type material. Being a II–VI semiconductor compound, group I and group V elements are considered suitable p-type dopants for ZnO. In this monograph, authors concentrate mostly on the different methods undertaken to achieve strong, reliable and stable p-type ZnO films. Ion implantation, being a suitable technique to achieve selective and localised doping, was used to dope ZnO with different group I and group V elements. Lithium and phosphorus ions were implanted using the conventional ion implantation technique, while plasma immersion ion implantation was used to implant phosphorus and nitrogen ions in the ZnO thin films. Such studies would bring interesting revelations in the optoelectronics field for the ultraviolet range and help to produce devices for the commercial market at a much cheaper cost.

The impact of time-varying phosphorus doping on ZnMgO thin films and achievement of dominant acceptor-bound-exciton peak

ZnO is a highly efficient and promising semiconductor material because of its large bandgap (3.37 eV) and exciton binding energy (60 meV). MgO also has a very high bandgap (7.8 eV), and the incorporation of Mg into ZnO can result in an alloy with a bandgap of more than 4 eV . We used plasma immersion ion implantation to dope phosphorus into Zn0.85Mg0.15O for achieving p-type ZnMgO. RF sputtering was used to deposit ZnMgO on a Si substrate. Phosphorus doping was conducted from 10 s to 70 s. Rapid thermal annealing of the samples was performed to remove any implantation defects. A highly dominant acceptor-bound-exciton peak was observed at 3.36 eV by photoluminescence measurements, which continued to dominate from low temperature to room temperature. Donor-bound acceptor and free-electron acceptor peaks were also observed at 3.24 eV and 3.28 eV, respectively.

Effects of Low Energy H- ion implantation on the Optical Properties of ZnMgO Thin Films

The optical and structural characteristics of H− ion-implanted ZnMgO were investigated by temperature-dependent photoluminescence (PL) and high-resolution X-ray diffraction (HRXRD). Low-energy (40 keV and 50 keV) hydrogen implantation was performed on RF-sputter-deposited ZnMgO thin films by varying the fluences from 1013 ions/cm2 to 5 × 1014 ions/cm2 . Highly c-axis-oriented <002> ZnO films were observed for all samples, as confirmed by HRXRD. A gradual decrease in the acceptor concentration was observed with increasing fluence, as confirmed by low-temperature PL results. This suggests that hydrogen atoms act as a shallow donor.

Effect of lithium-ion implantation of varying fluence on the optical properties of ZnMgO

The large bandgap (3.37 eV) and exciton binding energy (60 meV) makes ZnO most promising material in the area of optoelectronic devices. The efficiency of these devices can be enhanced by increasing the bandgap of those materials which is possible by band-gap engineering. It has been found that incorporation of Mg can increase the bandgap of the alloy up to 4 eV and even more. We investigated the optical properties of Zn1-x MgxO film implanted by Li at low energy (40 KeV) with dosage of 5x1013 ions/cm2 and 1014 ions/cm2 respectively. Prior to implantation 150 nm Zn1-x MgxO (x=0.15) film was deposited on Si substrate followed by annealing at 650°C and 750°C. For dosage of 5x1013 ions/cm2 and 1014 ions/cm2 the low temperature (15K) and room temperature photoluminescence spectra is dominated by the emission of 3.66 eV which is the band gap energy of Zn1-xMgxO, shifts to 3.63 eV at higher dosage of ions. With increasing energy (50 KeV) this peak was revealed only at 5x1013 ions/cm2. At 1014 ions/cm2 no sign of this peak was visible. The splitting of conduction band and valence band into multiple sub-bands causes a transition between the subband of conduction band and sub-band of heavy-hole and an emission occurs at 3.58 eV referred as 11H. The existence of acceptor-bound exciton peak (A°X) around 3.33 eV and the presence of donor-to-acceptor-pair peak at 3.24 eV provide strong evidence of increased acceptor concentration.

Photoluminescence and hall study for the production and recovery of defects in phosphorus implanted ZnO films

ZnO (002) films of thickness 0.5 μm were grown on c-plane sapphire substrates by Pulsed Laser Deposition technique at 400°C in oxygen ambient of 75 mTorr and implanted with 8x1014 cm-2 dose of phosphorus ions of energy 50 keV (Sample A) followed by Rapid Thermal Annealing in Ar (Sample B) and oxygen atmospheres (Sample C) at 750°C (30 seconds). AFM images depicted root-mean-square roughness for Sample A (10.072 nm), B (9.314 nm) and C (4.9 nm). Room-temperature Hall study revealed n-type conductivity with carrier concentrations of 9.69x1019cm-3, 1.36x1020 cm-3, 4x1018 cm-3 and Hall mobility of 0.727 cm2/V-s, 12.44 cm2/V-s, 54.3 cm2/V-s for Sample A, B and C respectively. Possible reasons might be the formation of vacancy clusters by implantation for Sample A, production of oxygen vacancies with agglomeration of vacancy clusters and without further aggregation of vacancy clusters with higher annealing temperature for Sample B and C respectively. Dominance of donor-bound exciton peaks were found to be at 3.295 eV, 3.284 eV, 3.281 eV and 3.272 eV for unimplanted sample, Sample A, B and C respectively from roomtemperature photoluminescence study.