As employers have moved away from defined benefit pension plans, they are sweetening their 401(k) and other defined contribution plans, according to a new Aon Hewitt survey.

In 2001, just 45% of employers allowed employees to make pretax contributions to defined contribution plans immediately after beginning work. Now, more than three quarters — or 76% — of employers allow immediate contributions, according to an Aon Hewitt survey released Wednesday.

In addition, employers are enhancing their 401(k) plan matching contributions. For example, in 2013, 19% of employers matched 100% of employees' 401(k) plan salary deferrals, up from 10% in 2011 and just 4% in 2001.

That design — matching 100% of employees' salary deferrals up to 6% of pay — has become the most common employer design, easily eclipsing what had been since 2001 the most popular employer match: matching 50% of employees' salary deferrals, up to 6% of pay. In 2013, 15% of employers offered such a match, down from 21% in 2001.

The sweetening of 401(k) and other defined contribution plans comes as many employers, concerned about the difficulty in predicting future costs, have moved away from defined benefit plans, such as by freezing benefit accruals for current plan participants and not allowing new employees to join the plan.

That has meant defined contribution plans have become the sole employment-based retirement plan for a steadily growing number of employees, and increasing the importance of the plans as a source of retirement income.

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“Employers are trying to make sure that employees have something for retirement,” said Rob Austin, director of retirement research in Aon Hewitt's Charlotte, N.C., office, referring to the growth of immediate defined contribution plan enrollment and the sweetening of employer matching contributions.

The survey of more than 400 defined contribution plan sponsors also found a big leap in the percentage of employers offering Roth 401(k) plans. Fifty percent of employers offer the plans, in which employees make after-tax contributions but, so long as certain conditions are met, can withdraw contributions and investment income tax-free. In 2006, just 11% of employers offered Roth 401(k) plans.

Roth 401(k) plans are appealing to employees who are in low tax brackets now but anticipate being in much higher tax brackets when they retire and can take tax-free distributions from the plans.

“Plan sponsors are enthusiastic about making the Roth provisions available to their workforce and allowing them to benefit from any tax advantages,” Mr. Austin said in a statement.