Impact of Direct Plasma Hydrogenation on Thermal Donor Formation in n-Type CZ Silicon

Abstract

In this paper, the role of a direct plasma hydrogenation treatment and a subsequent air annealing at 450°C in the formation of thermal donors ͑TDs͒ in n-type Czochralski ͑CZ͒ silicon is investigated by means of a combination of capacitance-voltage ͑C-V͒ and deep-level transient spectroscopy ͑DLTS͒. The hydrogenation treatment is found to enhance the introduction rate of TDs at 450°C for shorter annealing times, reaching its maximum acceleration after 5 h and for the longest plasma hydrogenation studied ͑2 h͒. For much longer annealing times, the TD introduction rate becomes independent of the presence of hydrogen in the material. DLTS detects only one donor level having an activation energy, which lowers with increasing doping density of the material, from 0.106 to 0.093 eV. After activation energy correction for the Poole-Frenkel electric-field-enhanced emission, this trap is found to fit well with the conventional singly ionized oxygen thermal donor level. However, from C-V free carrier and DLTS trap concentrations, it is derived that other shallower donors, created by the plasma hydrogenation and 450°C anneal should play an important role in the free carrier concentration increase of the n-CZ silicon.By now, it is a well-accepted fact that hydrogen plays a catalytic role in the formation of thermal donors in Czochralski ͑CZ͒ silicon. 1-4 Hydrogen can be introduced in several ways in the material, among which a hydrogen plasma treatment is of strong interest due to its low cost, high throughput, and low thermal budget. 5 Combining a low-temperature plasma hydrogenation, followed by a relatively short thermal donor anneal at 450°C, it has been demonstrated that p-type starting material can be converted into n-type silicon, and therefore, deep p-n junctions can be obtained in a fast and cheap way. 5,6 The depth of the junction is thereby controlled by the hydrogen in-diffusion profile. 6 So far, most of our studies have focused on p-type CZ starting material. 5,6 In this paper, the impact of the plasma hydrogenation time on the thermal donor ͑TD͒ formation during annealing at 450°C from 30 min up to 50 h in n-type CZ material is investigated. It is shown by capacitance-voltage ͑C-V͒ doping profiles that there is indeed an enhancement of the TD formation rate for annealing times t a450 р 5 h. The maximum effect is observed after a 5 h anneal and increases with increasing plasma hydrogenation time t H , i.e., with the total amount of hydrogen in the material. For longer t a450 , the hydrogen impact diminishes, resulting in a final free electron introduction rate from C-V of about 1.7-1.8 ϫ 10 14 cm Ϫ3 /h for the n-type CZ silicon studied. In order to identify the created donors, deep level transient spectroscopy ͑DLTS͒ was performed on a selected group of samples. The resulting spectra are dominated by a donor level at an energy of 0.093-0.106 eV below the conduction band E C , shifting to lower values for increasing n-type doping density. By taking into account the Poole-Frenkel barrier lowering with increasing el...

What Do We Know about Hydrogen-Induced Thermal Donors in Silicon?

“…First, the C-V results obtained on n-type Si will be briefly summarized; more detailed results have been published before. [29][30][31] More attention will be given here to the data for p-type material. When comparing the impact of the doping type, a general conclusion is that the introduction rate of TDs is significantly faster in pthan in n-type Si.…”

“…Hydrogenation-induced donor formation in n-type silicon.-The donor profiles in plasma-hydrogenated and 450°C annealed standard 5 ⍀ cm n-type CZ Si exhibit typically a decay toward the surface. 30 At ϳ1 m from the exposed surface, the carrier concentration levels off, forming a constant plateau. On the basis of these plateau values, one can demonstrate that heat-treatment at 450°C increases the free electron concentration from the starting value of ϳ9 ϫ 10 14 cm −3 to Ͼ 10 16 cm −3 after 50 h anneal.…”

“…On the basis of these plateau values, one can demonstrate that heat-treatment at 450°C increases the free electron concentration from the starting value of ϳ9 ϫ 10 14 cm −3 to Ͼ 10 16 cm −3 after 50 h anneal. 30 In addition, one can derive that plasma hydrogenation speeds up the donor formation rate, especially for shorter annealing times at 450°C. Longer hydrogenations also yield higher electron concentrations for the same annealing time ͑Fig.…”

“…2͒. 30 In order to determine more clearly the impact of the plasmahydrogenation treatment, Fig. 3 depicts the TD enhancement factor defined by 30 t H ͑t a ͒ = n t H ͑t a ͒ − n 0 ͑t a ͒ n 0 ͑t a ͒ ͓1͔…”

“…The materials indicated in Table I have been hydrogen-plasma treated in a plasma-enhanced chemical vapor deposition parallelplate system at a substrate temperature of typically 260-270°C for times ranging from 30 min up to 12 h. [29][30][31][32] The interstitial oxygen concentration ͓O i ͔ was measured by Fourier transform infrared ͑FTIR͒ absorption spectroscopy. Prior to the hydrogen-plasma exposure, the silicon wafers are ultrasonically cleaned in acetone and methanol, and rinsed in deionized water to remove the organic remains on the silicon surface.…”