Shiming Ang

Generation of amber III-nitride based light emitting diodes by indium rich InGaN quantum dots with InGaN wetting layer and AlN encapsulation layer

Indium rich InGaN nanostructures grown by metalorganic chemical vapor deposition were incorporated in InGaN/GaN quantum wells for long wavelength generation. These results were achieved by optimizing the growth temperature of the nanostructures, InGaN quantum well, the AlN capping layer and the GaN barrier layers. Before the growth of nanostructures, a thin InGaN wetting layer was included to reduce the lattice mismatch as well as to enhance the deposition of indium-rich InGaN nanostructures These individual quantum wells were each subsequently capped with an AlN layer which better preserved the In-rich phase in the nanostructures and prevented the indium interdiffusion between the InGaN/GaN heterojunctions. The AlN capping layer also reduces the effect of piezeoelectric field in the active layers of the light emitting diodes as seen from the reduction in the blueshift in the electroluminescence peaks with higher injection currents. The energy band profile of such a structure is discussed.

Dedicated Servo Recording System and Performance Evaluation

The perpendicular magnetic recording (PMR) in hard disk drives is approaching its physical limitation. The emerging technologies, such as heat assisted magnetic recording and microwave assisted magnetic recording have been proposed to record on magnetic media with thermally stable smaller size grains at higher areal density (AD). However, in the media fabrication, achieving well-isolated small size of grains is more challenging than obtaining high Ku material as recording media. Reducing the number of grains per bit is a major path for keeping AD growth of PMR in recent years. To minimize the SNR penalty at a smaller grain number per bit, pushing more on track density is the right approach. With the 2-D magnetic recording (TDMR) readers for inter-track interference cancellation, the off-track read capability is improved significantly for allowing a narrower track read. In the drive working environment, when the external vibration or other mechanical disturbance happens during the writing process, it creates more track squeeze at adjacent tracks and leaves a very narrow track at some locations of the track. When the track width is narrower than the squeeze to death width in the 747 curve, it causes hard failure in the channel. To solve the track squeeze problem, this paper proposes to add an additional magnetic recording layer in between the data recording layer and the soft underlayer of conventional PMR media. This additional recording layer is used to record servo information only. The continuous positioning error signal is able to improve the servo performance and to provide the real-time monitoring of the positioning error. When it is under bad servo conditions, the writing process can be stopped to avoid nontolerable track squeeze. The continuous servo signals are designed to be of moderate intensity at very low frequency, and its impact on data signal has been minimized. The linear density gap between the dedicated servo media and the conventional PMR media is able to be controlled within 3%. As the dedicated servo system keeps only around 100 wedges of track ID and sector ID at the data layer, the surface area saving at the data layer can break even in capacity. The dedicated servo technology together with TDMR readers is the key technology to achieve ultrahigh track density during both writing and reading processes.

Additional Reader Noise in TDMR Reader for ITI Cancelation

Shingled writing eliminates the writer width limitation and enables further bit aspect ratio reduction to extend the areal density growth of current perpendicular magnetic recording. The achievable track density of shingled magnetic recording (SMR) is determined by the reader width rather than the writer width. However, continuously scaling of reader width increases the reader resistance and results in higher reader noise. It would be an issue to maintain enough signal-to-noise ratio of reader. The 2-D magnetic recording (TDMR) was proposed to extend the density growth by advanced signal processing technique. In TDMR configuration, there are multiple readers to read multiple tracks simultaneously. Recently, the TDMR reader was proposed as a solution of intertrack interference cancelation, which allows to use wider reader to reproduce signals from narrow tracks. In this paper, an additional reader noise is observed when the reader was positioned to read two tracks at the same time. The formation of the reader sensor appears to be closely related to the current reader structure with hard bias magnet. This issue highlights that a solution is required to enable TDMR, probably a reader structure without hard-biased.

Performance of perpendicular magnetic recording with track squeeze using bidirectional pattern-dependent noise prediction detector

In this study, we investigated the performance of perpendicular magnetic recording with squeezed tracks. Based on spinstand signals, we developed a signal generation module to generate the readback signals for tracks of various squeezed track width and bit length. To mitigate the drop in bit error rate (BER) as squeezed track width decreases, we implemented the bidirectional pattern-dependent noise prediction (BiPDNP) detector with low-density parity check codes. To improve the BiPDNP detector over earlier work, we modified the log-likelihood ratio computation. Our results show that our proposed BiPDNP detector improves the bit error rate, and allows smaller squeezed track width and smaller bit length for the same BER performance.

Signal Processing for Dedicated Servo Recording System

Perpendicular magnetic recording (PMR) in the hard disk drive is approaching its physical limits. In an earlier work, the dedicated servo (DS) recording system has been proposed to provide continuous position error signal for servo, enable higher servo sampling rate, and improve the overall servo performance. A further benefit is that the DS layer results in surface area savings at the data layer. However, it was also reported that the embedded servo layer introduces baseline variation and non-linear transition shift (NLTS) to the readback signal of the data layer. In this paper, we propose novel signal processing techniques to improve the bit error rate (BER) in DS recording. The synchronous averaging technique is proposed to improve the BER in the presence of baseline variation distortions. Further, the servo and data-dependent noise prediction method is proposed to mitigate the effect of the NLTS. Through the use of these techniques the linear density loss from the conventional PMR media is reduced.

Influence of Data Patterns on Reader Performance at Off-Track Reading

In this paper, we focus our attention on the performance of read heads under the off-track reading condition when the reader is under the influence of the recorded magnetization patterns on the medium, and analyze how the magnetic field due to various data patterns impacts the read head behavior. The analysis is based on the micromagnetic modeling of the state of magnetization in read sensor considering its external magnetic fields due to both the hard bias and the media magnetization pattern. The effects of various magnetization patterns on media are analyzed. The effect of thermal magnetic agitation of the gyromagnetical precession of magnetizations is also evaluated. It is shown that to account for such effect is important for evaluation of magnetic recording schemes for extremely high density.